Remove obsolete comment.

The lock level for adding a parent table is now ShareUpdateExclusiveLock;
see commit fbcf4b92.  This comment didn't
get updated to match, but it doesn't seem important to mention this detail
here, so rather than updating it now, just take it out.

src/backend/commands/tablecmds.c

@@ -2676,8 +2676,7 @@ AlterTableGetLockLevel(List *cmds)
 				 * started before us will continue to see the old inheritance
 				 * behaviour, while queries started after we commit will see
 				 * new behaviour. No need to prevent reads or writes to the
-				 * subtable while we hook it up though. In both cases the
-				 * parent table is locked with AccessShareLock.
+				 * subtable while we hook it up though.
 				 */
 			case AT_AddInherit:
 			case AT_DropInherit: