Remove collapsibleNav module

* Remove vector-collapsiblenav preference.
  Usage on Wikimedia wikis:
  - 0.008% on en.wikipedia
  - 0.02% on de.wikipedia

* Moved module to MediaWiki core in I87149d7e15931f02ab70.

Bug: 46512
Change-Id: I47950b5375cae38e249af2e01afe1c4e5b1d10e4
Merge "Remove collapsibleNav module"