path: replaced parent_path() with g_path_get_dirname()

Again, GLib's version is more robust than ours.

src/path.h

@@ -58,16 +58,6 @@ char *pfx_dir(char *dst,
 /* relative playlist path to absolute playlist path */
 char *rpp2app_r(char *dst, const char *rel_path);
 
-/*
- * parent_path - saner version of dirname(3) with slightly different semantics
- *  - we will return "" instead of "." or "/" if we have no parent
- *    this is because we only deal with internal paths
- *  - we always skip over redundant slashes in the middle, if there are any
- *  - we will never get meaningful paths with trailing slashes in our callers
- *    (set handle_trailing_slashes to true if we do)
- */
-char *parent_path(char *path_max_tmp, const char *path);
-
 /* strips extra "///" and leading "/" and trailing "/" */
 char *sanitizePathDup(const char *path);