Revert 13208:
SSMAP.withDefault needs to be non-static,
because there are callers that rely on the fact
that it uses the current SSAMap instance as base,
and not an empty instance.
These callers were broken by that commit.

Also the non-static method has greater usability,
because it is more flexible,
and for the "empty SSAMap with default" case,
one just has to write SSAMap.emptySSAMap().withDefault(...)".

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13229 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c