[SecurityBundle] Get rid of sulu_security.user_manager service

<a href="https://github.com/danrot"><img height="50" width="50" src="https://avatars.githubusercontent.com/u/405874?v=2" /></a>
By @danrot _from_ https://github.com/sulu-cmf/SuluSecurityBundle/issues/31

This service injects the `security.context`-service, which causes some maintainability problems.
