Revert "Fix psql's single transaction mode on client-side errors with -c/-f switches".

This reverts commits a04ccf6df et al. in the back branches only.
There was some disagreement already over whether to back-patch
157f8739a, on the grounds that it is the sort of behavioral
change that we don't like to back-patch.  Furthermore, it now
looks like the logic needs some more work, which we don't have
time for before the upcoming 14.4 release.  Revert for now, and
perhaps reconsider later.

Discussion: https://postgr.es/m/17504-76b68018e130415e@postgresql.org

doc/src/sgml/ref/psql-ref.sgml

@@ -591,10 +591,8 @@ EOF
         <application>psql</application> to issue a <command>BEGIN</command> command
         before the first such option and a <command>COMMIT</command> command after
         the last one, thereby wrapping all the commands into a single
-        transaction. If any of the commands fails, a
-        <command>ROLLBACK</command> command is sent instead. This ensures that
-        either all the commands complete successfully, or no changes are
-        applied.
+        transaction.  This ensures that either all the commands complete
+        successfully, or no changes are applied.
        </para>
 
        <para>

src/bin/psql/startup.c

@@ -411,9 +411,7 @@ main(int argc, char *argv[])
 
 		if (options.single_txn)
 		{
-			res = PSQLexec((successResult == EXIT_SUCCESS) ?
-						   "COMMIT" : "ROLLBACK");
-			if (res == NULL)
+			if ((res = PSQLexec("COMMIT")) == NULL)
 			{
 				if (pset.on_error_stop)
 				{