Remove input buffer flush on sigwinch in index.

I recall seeing a message on IRC, where someone reported their macro
suddenly aborting after invoking an editor or something similar. At
the time, I wasn't sure of the problem, but this seems a likely
culprit, if the event caused a sigwinch.

Neither the menu.c nor the pager.c SigWinch handlers do such a thing.
The line was part of the initial import 21 years ago, so there is no
justification given.

curs_main.c

···
       if (SigWinch)
       {
 	SigWinch = 0;
-	mutt_flushinp ();
 	mutt_resize_screen ();
 	menu->top = 0; /* so we scroll the right amount */
 	/*