[nanvix] E: Remove lxml built-in shim from cpython

[esaurez]

Summary

Removes the lxml built-in shim from CPython entirely. lxml is a third-party package, not a CPython stdlib module, and the way it was integrated on Nanvix diverges from how upstream CPython loads third-party extensions. This PR aligns Nanvix with upstream: CPython should contain no lxml at all.

How upstream CPython handles lxml

It doesn't — lxml is third-party. On Linux, pip install lxml builds native CPython extension modules (lxml/etree.cpython-<plat>.so in site-packages, carrying DT_NEEDED for libxml2/libxslt), which CPython's standard ExtensionFileLoader dlopens. Zero CPython modifications, no Setup.local entry, no C trampoline, no Python bridge.

The band-aid being removed

Because makesetup cannot express dotted module names, Nanvix registered the Cython output under the flat name _lxml_etree via Modules/Setup.local, fronted by a C trampoline (lxml_etree_builtin.c) forwarding PyInit, plus a Python bridge (lxml/etree.py) re-exporting it under the dotted lxml.etree name, plus a separate system shared library (liblxml_etree.so). That is two artifacts where Linux has one, and several glue layers that exist only to route a third-party package through the stdlib build machinery.

What this restores

lxml returns the upstream way once the nanvix/lxml port emits native CPython extension modules into site-packages — at which point CPython's standard importer loads lxml.etree directly with zero CPython-side changes. This is tracked as a follow-up change to the nanvix/lxml port repo.

Removed

• Modules/lxml_etree_builtin.c, Modules/lxml_elementpath_builtin.c (the flat-name PyInit trampolines).
• .nanvix/setup_local.py: _lxml_etree / _lxml_elementpath entries.
• .nanvix/lxml.py: deleted. Its generate_setup_local() (general Setup.local generation, not lxml-specific despite the file name) is moved into .nanvix/setup_local.py next to render_setup_local(); the unused clear_setup_local() and the lxml runtime staging (stage_lxml_runtime / _ETREE_SHIM) are dropped.
• .nanvix/build.py / package.py / test.py: drop the lxml module import, the lxml runtime staging calls, and the standalone lxml.etree import/parse smoke snippet.
• .nanvix/z.py: drop lxml / libxml2 / libxslt from _DEP_EXPECTED_LIBS and the now-dead python-packages/ payload extraction (only lxml used it; the native-extension form will not).
• .nanvix/nanvix.toml: drop libxml2 / libxslt / lxml dependency pins.
• .nanvix/config.py: drop test_nanvix_lxml from the regrtest list.
• Lib/test/test_nanvix_lxml.py: deleted.

Base / relationship to other PRs

• Base: feat/wave5-pr-a-stdlib-so ([nanvix] E: Build stdlib extensions as .so #732). Independent of the _ssl/_hashlib/_ctypes externalization in [nanvix] E: Build _ssl/_hashlib/_ctypes as .so #738 — this is a sibling cleanup, not stacked on it.
• [nanvix] E: Build _ssl/_hashlib/_ctypes as .so #738 was scoped down to drop its lxml content (lxml reverted there to its pre-existing static state); this PR then removes lxml outright.

Validation

• Modules/Setup.local renders cleanly with exactly one *static* and one *shared* marker, no lxml entries.
• ./z lint (black + pyright) clean.
• pre-commit run clean on all changed files.