Remove test for COMMENT ON DATABASE

Our current DDL only allows a database name to be specified in COMMENT
ON DATABASE, which Andrew Dunstan reports to make this test fail on the
buildfarm.  Remove the line until we gain a DDL command that allows the
current database to be operated on without having the specify it by
name.

Backpatch to 9.5, where these tests appeared.

Discussion: https://postgr.es/m/e6084b89-07a7-7e57-51ee-d7b8fc9ec864@2ndQuadrant.com

src/test/modules/test_ddl_deparse/expected/comment_on.out

@@ -21,5 +21,3 @@ COMMENT ON TRIGGER trigger_1 ON datatype_table IS 'TRIGGER test';
 NOTICE:  DDL test: type simple, tag COMMENT
 COMMENT ON RULE rule_1 IS 'RULE test';
 NOTICE:  DDL test: type simple, tag COMMENT
--- should not fire
-COMMENT ON DATABASE contrib_regression IS 'contrib regression';

src/test/modules/test_ddl_deparse/sql/comment_on.sql

@@ -12,6 +12,3 @@ COMMENT ON VIEW datatype_view IS 'This is a view';
 COMMENT ON FUNCTION c_function_test() IS 'FUNCTION test';
 COMMENT ON TRIGGER trigger_1 ON datatype_table IS 'TRIGGER test';
 COMMENT ON RULE rule_1 IS 'RULE test';
-
--- should not fire
-COMMENT ON DATABASE contrib_regression IS 'contrib regression';