Make proofs optional for Z3.

I am not sure whether the _synchronized_ block
is needed for Z3 creation (as further operations are unsychronized).
Without the static synchronized block an extra static class
is unnecessary.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@15134 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c