Remove custom grains module from 2018.3 branch

Support for passing the grains data to custom grains modules wasn't
added until the 2019.2 release cycle. It seems like this file was
included when something else was backported to 2018.3.

tests/integration/files/file/base/_grains/custom_grains.py

@@ -1,5 +0,0 @@
-# -*- coding: utf-8 -*-
-
-
-def test(grains):
-    return {'custom_grain_test': 'itworked' if 'os' in grains else 'itdidntwork'}