[python] Fix incorrect VIRTUAL_ENV in pyenv-mode-set-local-version (#17246)

Remove the erroneous `(setenv "VIRTUAL_ENV" version)` call from
`spacemacs//pyenv-mode-set-local-version`.

This line was introduced in commit e699763 to help Python LSP servers
detect pyenv version changes. However, the implementation was incorrect:
it set `VIRTUAL_ENV` to a bare version string (e.g. "3.13.2") instead
of a filesystem path. The `VIRTUAL_ENV` environment variable, by
convention, should contain the absolute path to a virtualenv directory.

This causes issues in pyenv + Poetry workflows: Poetry reads
`VIRTUAL_ENV` and interprets the version string as a virtualenv path,
leading to errors. The severity depends on hook execution order — if
`spacemacs//python-setup-backend` (which calls `poetry-venv-workon` →
`pyvenv-activate`) runs after the pyenv hook, the bogus value gets
overwritten and the bug is masked; otherwise Poetry breaks.

The `setenv` call is also unnecessary. `pyenv-mode-set` already handles
all required environment setup internally:
- calls `pythonic-activate` with the full path from `pyenv-mode-full-path`
  to set `python-shell-virtualenv-root`
- sets `PYENV_VERSION` to the version string

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: elvinzeng

layers/+lang/python/funcs.el

@@ -243,7 +243,6 @@ Equivalent to: autoflake --remove-all-unused-imports --in-place <FILE>"
                                        (line-beginning-position)
                                        (line-end-position)))))))
     (cond ((member version (pyenv-mode-versions))
-           (setenv "VIRTUAL_ENV" version)
            (pyenv-mode-set version))
           (t (message "pyenv: version `%s' is not installed (set by %s)"
                       version file-path)))))