setup: drop `setup_git_env()`

setup: drop `setup_git_env()`

The `setup_git_env()` function is a trivial wrapper around
`setup_git_env_internal()` and has a single call site only. Drop the
function.

While at it, drop stale documentation in "environment.h" that points to
this function, even though it hasn't been exposed to callers outside of
"setup.c" since 43ad1047a9 (setup: stop using `the_repository` in
`setup_git_env()`, 2026-03-27) anymore.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>