Drop CVC4 support

[RyanGlScott]

The CVC4 repo has been deprecated and unmaintained for many years, and the CVC5 developers now recommend switching to CVC5 instead of using CVC4. We have kept a CVC4 build in what4-solvers for many years due to its previous popularity, but it is becoming increasingly difficult to maintain the CVC4 code as operating system requirements change (e.g., the vast majority of patches that we have to curate are CVC4-specific). I think it is time that we officially drop CVC4 from what4-solvers and only support CVC5 going forward.

Related issues:

• Drop CVC4 support cryptol#1806