Remove further usage of Class<?> of Formulas.
If we are going to cast to a subtype of Formula,
it is not worse to use instanceof checks than to get the Class<?>
and compare it.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13682 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c