SpecialEmailUser: Remove validate/get target without sender specified

SpecialEmailUser: Remove validate/get target without sender specified

This was deprecated in 1.30 and per usage below, these has been cleaned
up. Passing "null" is deprecated and no longer used anywhere. All usage
now pass a user object that is the sender.

Usage
=====

* validateTarget():
https://codesearch.wmflabs.org/search/?q=%5CbvalidateTarget%5Cb&i=nope&files=&repos=

* getTarget():
https://codesearch.wmflabs.org/search/?q=%5Cb(-%3E%7C%3A%3A)getTarget%5Cb&i=nope&files=&repos=

Change-Id: I22ed2bc42216712602f8b4685035c3396d3d1e5c