filter::stream: free the filter sanely

Don't use the filter's free callback to free the actual data structure
holding the filter, as we may not always actually initialize it (the
test may be skipped).

tests/filter/stream.c

@@ -25,6 +25,7 @@ void test_filter_stream__cleanup(void)
 	g_repo = NULL;
 
 	git_filter_unregister("compress");
+	git__free(compress_filter);
 }
 
 #define CHUNKSIZE 10240
@@ -123,11 +124,6 @@ static int compress_filter_stream_init(
 	return 0;
 }
 
-static void compress_filter_free(git_filter *f)
-{
-	git__free(f);
-}
-
 git_filter *create_compress_filter(void)
 {
 	git_filter *filter = git__calloc(1, sizeof(git_filter));
@@ -136,7 +132,6 @@ git_filter *create_compress_filter(void)
 	filter->version = GIT_FILTER_VERSION;
 	filter->attributes = "+compress";
 	filter->stream = compress_filter_stream_init;
-	filter->shutdown = compress_filter_free;
 
 	return filter;
 }