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

This command declares a SQL identifier for a SQL statement to be used in other
embedded SQL statements. The identifier is linked to a connection.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Shawn Wang <shawn.wang.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/TY2PR01MB24438A52DB04E71D0E501452F5630@TY2PR01MB2443.jpnprd01.prod.outlook.com

This reverts commit bd7c95f0,
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

DECLARE STATEMENT is a statement that lets users declare an identifier
pointing at a connection.  This identifier will be used in other embedded
dynamic SQL statement such as PREPARE, EXECUTE, DECLARE CURSOR and so on.
When connecting to a non-default connection, the AT clause can be used in
a DECLARE STATEMENT once and is no longer needed in every dynamic SQL
statement.  This makes ECPG applications easier and more efficient.  Moreover,
writing code without designating connection explicitly improves portability.

Authors: Ideriha-san ("Ideriha, Takeshi" <ideriha.takeshi@jp.fujitsu.com>)
         Kuroda-san ("Kuroda, Hayato" <kuroda.hayato@jp.fujitsu.com>)

Discussion: https://postgr.es/m4E72940DA2BF16479384A86D54D0988A565669DF@G01JPEXMBKW04