User: Remove deprecated methods ::getGroupPage() & ::makeGroupLinkHTML()

User: Remove deprecated methods ::getGroupPage() & ::makeGroupLinkHTML()

These methods were deprecated in 1.29 and usage can no longer be found so,
removing ahead of 1.34 cut.

Usage
=====

* User::getGroupPage() -
  https://codesearch.wmflabs.org/search/?q=%5B%5E%3E%5D(User)%3A%3AgetGroupPage%5C(&i=nope&files=&repos=

* User::makeGroupLinkHTML() -
  https://codesearch.wmflabs.org/search/?q=%5B%5E%3E%5D(User)%3A%3AmakeGroupLinkHTML%5C(&i=nope&files=&repos=

Bug: T220656
Change-Id: I4e48eb351883b044269d77a8baaf7b5030138c34