libpq: Remove PQservice()

This routine has been introduced as a shortcut to be able to retrieve a
service name from an active connection, for psql.  Per discussion, and
as it is only used by psql, let's remove it to not clutter the libpq API
more than necessary.

The logic in psql is replaced by lookups of PQconninfoOption for the
active connection, instead, updated each time the variables are synced
by psql, the prompt shortcut relying on the variable synced.

Reported-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250706161319.c1.nmisch@google.com
Backpatch-through: 18

src/interfaces/libpq/exports.txt

@@ -205,9 +205,8 @@ PQcancelFinish            202
 PQsocketPoll              203
 PQsetChunkedRowsMode      204
 PQgetCurrentTimeUSec      205
-PQservice                 206
-PQsetAuthDataHook         207
-PQgetAuthDataHook         208
-PQdefaultAuthDataHook     209
-PQfullProtocolVersion     210
-appendPQExpBufferVA       211
+PQsetAuthDataHook         206
+PQgetAuthDataHook         207
+PQdefaultAuthDataHook     208
+PQfullProtocolVersion     209
+appendPQExpBufferVA       210

src/interfaces/libpq/fe-connect.c

@@ -7461,14 +7461,6 @@ PQdb(const PGconn *conn)
 	return conn->dbName;
 }
 
-char *
-PQservice(const PGconn *conn)
-{
-	if (!conn)
-		return NULL;
-	return conn->pgservice;
-}
-
 char *
 PQuser(const PGconn *conn)
 {

src/interfaces/libpq/libpq-fe.h

@@ -400,7 +400,6 @@ extern int	PQrequestCancel(PGconn *conn);
 
 /* Accessor functions for PGconn objects */
 extern char *PQdb(const PGconn *conn);
-extern char *PQservice(const PGconn *conn);
 extern char *PQuser(const PGconn *conn);
 extern char *PQpass(const PGconn *conn);
 extern char *PQhost(const PGconn *conn);