riotbuild/requirements.txt: remove ed25519

[crasbe]

The ed25519 dependency has been added in #73 for SUIT draft-moran-v4 in RIOT-OS/RIOT#11818, however the support for that has been removed in RIOT-OS/RIOT#13486.

Since then the ed25519 package was not required anymore and now causes build issues with the update to Ubuntu 26.04 LTS.
The ed25519 package is not maintained anymore and depends on versioneer which is not maintained anymore either and causes a build failure:

65.98 Collecting ed25519==1.4 (from -r /tmp/requirements.txt (line 37))
66.00   Downloading ed25519-1.4.tar.gz (866 kB)
66.01      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 866.4/866.4 kB 128.0 MB/s eta 0:00:00
66.04   Installing build dependencies: started
66.62   Installing build dependencies: finished with status 'done'
66.62   Getting requirements to build wheel: started
66.91   Getting requirements to build wheel: finished with status 'error'
66.91   error: subprocess-exited-with-error
66.91
66.91   × Getting requirements to build wheel did not run successfully.
66.91   │ exit code: 1
66.91   ╰─> [33 lines of output]
66.91       /tmp/pip-install-x25yk91m/ed25519_c9b138c359ca4d269e2b8eb0b9986b1f/versioneer.py:587: SyntaxWarning: "\s" is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\s"? A raw string is also an option.
66.91         mo = re.search(r'=\s*"(.*)"', line)
66.91       Traceback (most recent call last):
66.91         File "/local/python-riot/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
66.91           main()
66.91           ~~~~^^
66.91         File "/local/python-riot/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 373, in main
66.91           json_out["return_val"] = hook(**hook_input["kwargs"])
66.91                                    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
66.91         File "/local/python-riot/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 143, in get_requires_for_build_wheel
66.91           return hook(config_settings)
66.91         File "/tmp/pip-build-env-ianufgey/overlay/lib/python3.14/site-packages/setuptools/build_meta.py", line 333, in get_requires_for_build_wheel
66.91           return self._get_build_requires(config_settings, requirements=[])
66.91                  ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
66.91         File "/tmp/pip-build-env-ianufgey/overlay/lib/python3.14/site-packages/setuptools/build_meta.py", line 301, in _get_build_requires
66.91           self.run_setup()
66.91           ~~~~~~~~~~~~~~^^
66.91         File "/tmp/pip-build-env-ianufgey/overlay/lib/python3.14/site-packages/setuptools/build_meta.py", line 520, in run_setup
66.91           super().run_setup(setup_script=setup_script)
66.91           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
66.91         File "/tmp/pip-build-env-ianufgey/overlay/lib/python3.14/site-packages/setuptools/build_meta.py", line 317, in run_setup
66.91           exec(code, locals())  # noqa: S102 # exec is intentional here
66.91           ~~~~^^^^^^^^^^^^^^^^
66.91         File "<string>", line 104, in <module>
66.91         File "/tmp/pip-install-x25yk91m/ed25519_c9b138c359ca4d269e2b8eb0b9986b1f/versioneer.py", line 1405, in get_version
66.91           return get_versions()["version"]
66.91                  ~~~~~~~~~~~~^^
66.91         File "/tmp/pip-install-x25yk91m/ed25519_c9b138c359ca4d269e2b8eb0b9986b1f/versioneer.py", line 1339, in get_versions
66.91           cfg = get_config_from_root(root)
66.91         File "/tmp/pip-install-x25yk91m/ed25519_c9b138c359ca4d269e2b8eb0b9986b1f/versioneer.py", line 399, in get_config_from_root
66.91           parser = configparser.SafeConfigParser()
66.91                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
66.91       AttributeError: module 'configparser' has no attribute 'SafeConfigParser'. Did you mean: 'RawConfigParser'?