Add "skipping" to the NOTICE produced by DROP OPERATOR CLASS IF EXISTS.

This makes this message consistent with all the other similar notices
produced by other DROP IF EXISTS commands.

Noted by KaiGai Kohei

src/backend/commands/opclasscmds.c

@@ -1564,7 +1564,7 @@ RemoveOpClass(RemoveOpClassStmt *stmt)
 	if (!HeapTupleIsValid(tuple))
 	{
 		ereport(NOTICE,
-				(errmsg("operator class \"%s\" does not exist for access method \"%s\"",
+				(errmsg("operator class \"%s\" does not exist for access method \"%s\", skipping",
 						NameListToString(stmt->opclassname), stmt->amname)));
 		return;
 	}

src/test/regress/expected/drop_if_exists.out

@@ -205,7 +205,7 @@ NOTICE:  server "test_server_exists" does not exist, skipping
 DROP OPERATOR CLASS test_operator_class USING btree;
 ERROR:  operator class "test_operator_class" does not exist for access method "btree"
 DROP OPERATOR CLASS IF EXISTS test_operator_class USING btree;
-NOTICE:  operator class "test_operator_class" does not exist for access method "btree"
+NOTICE:  operator class "test_operator_class" does not exist for access method "btree", skipping
 DROP OPERATOR CLASS test_operator_class USING no_such_am;
 ERROR:  access method "no_such_am" does not exist
 DROP OPERATOR CLASS IF EXISTS test_operator_class USING no_such_am;