LATX, clean: reduce the LATX tree to the maintained x86-on-LoongArch scope

[LaurenIsACoder]

Summary

This PR prunes code and vendored assets that are outside the maintained LATX scope.

The current LATX tree only builds and runs x86 user-mode targets (i386-linux-user and x86_64-linux-user) on a LoongArch host, but the repository still carried a
large amount of unused multi-architecture code inherited from the original QEMU/third-party layout. This change removes those unused paths and keeps the tree aligned
with what LATX actually builds and ships today.

What this changes

• remove unused default-configs entries for non-x86 targets
• remove unused linux-user target directories and non-LoongArch host helpers
• remove unused disassembler sources and non-LoongArch TCG backends
• prune vendored capstone_git content down to the X86 subset actually used by LATX
• keep build scripts aligned with the reduced tree

Why this is safe

The removed code is outside the currently maintained LATX execution scope:

• guest targets kept: i386-linux-user, x86_64-linux-user
• host backend kept: loongarch
• disassembler support kept for the active paths used by LATX

No supported LATX build target was intentionally removed.

Validation

Validated on a LoongArch host with:

• ./latxbuild/build32.sh -c
• ./latxbuild/build64.sh -c

Runtime smoke tests also passed with both rebuilt binaries:

• static busybox for i386
• static busybox for x86_64
• dynamic Debian 10 hello for i386
• dynamic Debian 10 hello for x86_64

Notes

This is a cleanup-only change. It does not introduce new translation features or change the supported LATX target set.