Scrap the cancel button from the toolbar

It's unnecessary, big and ugly, way too close to an actually useful button, and
adds oodles of complexity to the mire that is VPT without significant user benefit
whilst taking up an unjustifiably huge amount of the already-over-burdened space.
This means that the minimum browser width for English on default Vector to show
the toolbar correctly is now 945px, down from 1029px.

Bonuses: fix MWCancelConfirmDialog's cancel button to be destructive (it is) and
remove a now-recently-fixed comment asking for the save button to be styled using
a generic class (provided by the constructive flag).

Change-Id: I30dc7020121f0dd6907b61ef674a7cb14eb00652