Simplify the FormulaManagerView package:
After removing the "replacing" subpackage,
we have not need anymore for some of its extendability,
so we can remove LoadManagers.
Also merge AbstractBaseManagerView and BaseManagerView.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14287 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c