run-command.c: retire unused run_hook_with_custom_index()

run-command.c: retire unused run_hook_with_custom_index()

This was originally meant to be used to rewrite run_commit_hook()
that only special cases the GIT_INDEX_FILE environment, but the
run_hook_ve() refactoring done earlier made the implementation of
run_commit_hook() thin and clean enough.

Nobody uses this, so retire it as an unfinished clean-up made
unnecessary.

Signed-off-by: Junio C Hamano <redacted>