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 <gitster@pobox.com>

run-command.c

@@ -792,20 +792,3 @@ int run_hook_le(const char *const *env, const char *name, ...)
 
 	return ret;
 }
-
-int run_hook_with_custom_index(const char *index_file, const char *name, ...)
-{
-	const char *hook_env[3] =  { NULL };
-	char index[PATH_MAX];
-	va_list args;
-	int ret;
-
-	snprintf(index, sizeof(index), "GIT_INDEX_FILE=%s", index_file);
-	hook_env[0] = index;
-
-	va_start(args, name);
-	ret = run_hook_ve(hook_env, name, args);
-	va_end(args);
-
-	return ret;
-}