Alembic import: prevent double user decrement when cancelling import.

BKE_libblock_free_us() was called on the object data, which decrements
its user count, after which the same function was called on the object,
which decrements the user count of the object data again. This double
decrement was too much.

source/blender/alembic/intern/alembic_capi.cc

@@ -766,10 +766,6 @@ static void import_endjob(void *user_data)
 		for (iter = data->readers.begin(); iter != data->readers.end(); ++iter) {
 			Object *ob = (*iter)->object();
 
-			if (ob->data) {
-				BKE_libblock_free_us(data->bmain, ob->data);
-				ob->data = NULL;
-			}
 
 			BKE_libblock_free_us(data->bmain, ob);
 		}