environment.c: remove unused function

get_refs_directory() is not used anywhere.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -311,7 +311,6 @@ extern char *git_work_tree_cfg;
 extern int is_inside_work_tree(void);
 extern const char *get_git_dir(void);
 extern char *get_object_directory(void);
-extern char *get_refs_directory(void);
 extern char *get_index_file(void);
 extern char *get_graft_file(void);
 extern int set_git_dir(const char *path);