refactor: remove the dead CodecheckMetadataDAO and schema.xml

[nuest]

Three files described the codecheck_metadata table and none of them agreed.

• The migration creates version, publication_type, manifest, repository, source, codecheckers, certificate, issue, check_time, summary, report, additional_content.
• schema.xml declared opt_in, code_repository, data_repository, dependencies, execution_instructions.
• CodecheckMetadataDAO queried identifier, manifest_files, paper_metadata, repositories.

Only the migration is real, so the other two are removed.

CodecheckMetadataDAO was unreachable: nothing outside its own unit test referenced it, and every query would throw against the actual table — swallowed by a bare catch returning null or an empty array, so a caller would have seen "no data" rather than an error. CodecheckSubmissionDAO is the live one.

schema.xml was never loaded. OJS 3.5 installs plugin schemas through getInstallMigration(), which this plugin implements; Plugin::getInstallSchemaFile() is now a final, empty stub a plugin cannot override.

This drops the suite from 132 tests to 113. The 19 removed tests exercised the dead DAO against a mocked DB, so they passed while asserting nothing about code that could run.