cache: remove unused function 'have_git_dir'

This function was added in d2b0708 (2008-09-27, add have_git_dir()
function) as a preparation for adbc0b6 (2008-09-30, cygwin: Use native
Win32 API for stat).

However the second referenced commit was reverted in f66450a (2013-06-22,
cygwin: Remove the Win32 l/stat() implementation), so we don't need to
expose this wrapper function any more as a public API.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -391,7 +391,6 @@ extern int is_bare_repository(void);
 extern int is_inside_git_dir(void);
 extern char *git_work_tree_cfg;
 extern int is_inside_work_tree(void);
-extern int have_git_dir(void);
 extern const char *get_git_dir(void);
 extern int is_git_directory(const char *path);
 extern char *get_object_directory(void);

environment.c

@@ -155,11 +155,6 @@ int is_bare_repository(void)
 	return is_bare_repository_cfg && !get_git_work_tree();
 }
 
-int have_git_dir(void)
-{
-	return !!git_dir;
-}
-
 const char *get_git_dir(void)
 {
 	if (!git_dir)