Fix connection handling for DEALLOCATE and DESCRIBE statements

Fix connection handling for DEALLOCATE and DESCRIBE statements

After binding a statement to a connection with DECLARE STATEMENT the connection
was still not used for DEALLOCATE and DESCRIBE statements. This patch fixes
that, adds a missing warning and cleans up the code.

Author: Hayato Kuroda
Reviewed-by: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/TYAPR01MB5866BA57688DF2770E2F95C6F5069%40TYAPR01MB5866.jpnprd01.prod.outlook.com