libpkgconf: remove legacy check to ensure pkgconf_pkg_dir_list_build() is only called once.

this is not necessary anymore because:

- we do path entry deduplication
- it is possible that we want to prime the dir list with some user-specified paths prior to building it

libpkgconf/pkg.c

@@ -113,9 +113,6 @@ pkg_get_parent_dir(pkgconf_pkg_t *pkg)
 void
 pkgconf_pkg_dir_list_build(pkgconf_client_t *client, unsigned int flags)
 {
-	if (client->dir_list.head != NULL || client->dir_list.tail != NULL)
-		return;
-
 	pkgconf_path_build_from_environ("PKG_CONFIG_PATH", NULL, &client->dir_list, true);
 
 	if (!(flags & PKGCONF_PKG_PKGF_ENV_ONLY))