do not unset the ignoreNextMousedown option

There is a bug in android where multiple mousedown events are being
fired so we don't want this being cleared after the first one. We will
set this callback up once and just make sure it's returning the correct
thing always.

There is only 1 use of this that I can find and it's in the exact spot that
we want to leave it around. We couldn't find any uses of it in classroom.

src/services/mouse.ts

@@ -54,7 +54,6 @@ class Controller_mouse extends Controller_latex {
     (e.target as any).unselectable = true; // http://jsbin.com/yagekiji/1 // TODO - no idea what this unselectable property is
 
     if (cursor.options.ignoreNextMousedown(e)) return;
-    else cursor.options.ignoreNextMousedown = ignoreNextMouseDownNoop;
 
     // some elements should not act like internal mathquill nodes. Tokens for instance define external
     // click / hover behaviors. So we have mathquill act like the item was never clicked. This allows