Remove old Vector stuff

The Vector extension is being discontinued per bug 45051. Remove it,
as well as all settings that no longer exist or are the same as defaults.

Also remove settings for:
* $wgVectorUseSimpleSearch - true by default since 1.21 per mw.org
* $wgVectorUseIconWatch - true by default since 1.22 per mw.org
* $wgVectorShowVariantName - removed in 1.19 per mw.org
* $wmgEnableVector - setting $wgSkipSkins[] = 'vector' can be done
  manually if anything ever needs it

Notably did not remove:
* $wmgUseUsabilityInitiative, $wmgUsabilityEnforce - meh, someone else
  can fix up that mess some other time
* $wmgUseMicroDesign - nothing uses it, but I've heard some bold plans
  about micro design improvements, so I'm leaving this be to not get
  shouted at

Change-Id: I2b6c0e4e047848996d2047c9022390f7121f2571