tweaks: remove a redundant statement, a spurious reference to 'cutbottom'

This assignment should have been removed in commit f5ac8c1e from
six years ago, as it became superfluous then.  Later changes made
'cutbottom' become valid only when something had actually been cut.

src/text.c

@@ -1036,7 +1036,6 @@ void add_undo(undo_type action, const char *message)
 		break;
 	case PASTE:
 		u->cutbuffer = copy_buffer(cutbuffer);
-		u->tail_lineno += cutbottom->lineno - cutbuffer->lineno;
 		if (thisline == openfile->filebot)
 			u->xflags |= INCLUDED_LAST_LINE;
 		break;