Revert "Add temporary debugging assertion, in 9.4 branch only."

This reverts commit 5ee940e1.
Further debugging is needed, but it'll look different than this,
so for simplicity revert this first.

src/backend/commands/indexcmds.c

@@ -773,9 +773,6 @@ DefineIndex(Oid relationId,
 	UnregisterSnapshot(snapshot);
 	InvalidateCatalogSnapshot();
 
-	/* Temporary debug check: we should be advertising we have no snapshots. */
-	Assert(MyPgXact->xmin == InvalidTransactionId);
-
 	/*
 	 * The index is now valid in the sense that it contains all currently
 	 * interesting tuples.  But since it might not contain tuples deleted just