basic/conf-files: make conf_files_list() take just a single directory

This function had two users (apart from tests), and both only used one
argument. And it seems likely that if we need to pass more directories,
either the _nulstr() or the _strv() form would be used. Let's simplify
the code.

src/shared/bootspec.c

@@ -257,7 +257,7 @@ static int boot_entries_find(
         assert(entries);
         assert(n_entries);
 
-        r = conf_files_list(&files, ".conf", NULL, 0, dir, NULL);
+        r = conf_files_list(&files, ".conf", NULL, 0, dir);
         if (r < 0)
                 return log_error_errno(r, "Failed to list files in \"%s\": %m", dir);