Fix incorrect merge in ECPG code with DECLARE

The same condition was repeated twice when comparing the connection used
by existing declared statement with the one coming from a fresh DECLARE
statement.  This had no consequences, but let's keep the code clean.
Oversight in f576de1.

Author: Shenhao Wang
Discussion: https://postgr.es/m/OSBPR01MB42149653BC0AB0A49D23C1B8F2C69@OSBPR01MB4214.jpnprd01.prod.outlook.com
Backpatch-through: 14

src/interfaces/ecpg/preproc/ecpg.header

@@ -594,7 +594,6 @@ check_declared_list(const char *name)
 			continue;
 		if (strcmp(name, ptr -> name) == 0)
 		{
-			if (connection)
 			if (connection && strcmp(ptr->connection, connection) != 0)
 				mmerror(PARSE_ERROR, ET_WARNING, "connection %s is overwritten with %s by DECLARE statement %s", connection, ptr->connection, name);
 			connection = mm_strdup(ptr -> connection);