Remove unnecessary break in pg_logical_replication_slot_advance()

pg_logical_replication_slot_advance() included a break condition to stop
when a targeted LSN is reached, when processing a series of WAL records
with XLogReadRecord().  Since 38a957316d, it matched with the check of
its main while loop.  This condition saved from an extra CFI check,
actually pointless, so let's remove this condition and simplify the
code.

In passing, fix an incorrect comment.

Author: Bharath Rupireddy
Reviewed-by: Tom Lane, Gurjeet Singh
Discussion: https://postgr.es/m/CALj2ACWfGDLQ2cy7ZKwxnJqbDkO6Yvqqrqxne5ZN4HYm=PRTGg@mail.gmail.com

src/backend/replication/slotfuncs.c

@@ -500,7 +500,7 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 		/* invalidate non-timetravel entries */
 		InvalidateSystemCaches();
 
-		/* Decode at least one record, until we run out of records */
+		/* Decode records until we reach the requested target */
 		while (ctx->reader->EndRecPtr < moveto)
 		{
 			char	   *errm = NULL;
@@ -523,10 +523,6 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 			if (record)
 				LogicalDecodingProcessRecord(ctx, ctx->reader);
 
-			/* Stop once the requested target has been reached */
-			if (moveto <= ctx->reader->EndRecPtr)
-				break;
-
 			CHECK_FOR_INTERRUPTS();
 		}