Remove unused class ImmutableMapMerger. If we need it again,
it would better be rewritten using MergeConflictHandler,
and with a deterministic order of the resulting map.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14854 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c