Revert "Add DECLARE STATEMENT support to ECPG."

Revert "Add DECLARE STATEMENT support to ECPG."

This reverts commit bd7c95f0c1a38becffceb3ea7234d57167f6d4bf,
along with assorted follow-on fixes.  There are some questions
about the definition and implementation of that statement, and
we don't have time to resolve them before v13 release.  Rather
than ship the feature and then have backwards-compatibility
concerns constraining any redesign, let's remove it for now
and try again later.

Discussion: https://postgr.es/m/TY2PR01MB2443EC8286995378AEB7D9F8F5B10@TY2PR01MB2443.jpnprd01.prod.outlook.com