Move close method from FormulaManagerView to Solver.
The latter is the main class that is instantiated,
so it should also be the one that is closed.
This also lets us closed all formula managers if we have instantiated
more than one, which was not done previously.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@15031 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c