Have zoomTo make setEnd = -1

There was a bug which could be reproduced like this:
0. Open IGV
1. Enter a short locus, say chr1:100, zooming almost all the way in
2. Click somewhere else on the cytoband panel, changing to that location
3. Press the zoom out button

Expected result:
zoom to level 22, from 23 (or something like that)

Actual result:
zoom went nearly all the way out, level 1 or 2

Reason being that the scale was being calculated really wrong because setEnd was set in step 1 above but never cleared
This should clear in when appropriate (I hope)
Remove duplicate zoom calculation