Fix detection of CryptoMinisat4. A FindCryptoMiniSat4 CMake module
is not needed because cryptominisat4 exports its targets that can
be picked up by using find_package() in config mode.

If the CryptoMiniSat4 cannot be found the user will see a message
like this...

```
CMake Warning at CMakeLists.txt:224 (find_package):
  Could not find a package configuration file provided by "cryptominisat4"
  with any of the following names:

    cryptominisat4Config.cmake
    cryptominisat4-config.cmake

  Add the installation prefix of "cryptominisat4" to CMAKE_PREFIX_PATH or set
  "cryptominisat4_DIR" to a directory containing one of the above files.  If
  "cryptominisat4" provides a separate development package or SDK, be sure it
  has been installed.
```

The user can fix this by setting the cryptominisat4_DIR option
(e.g. cmake -Dcryptominisat4_DIR:STRING=/path/to/cryptominisat4/build).

This should be set the directory containing the ``cryptominisat4Config.cmake``
which is available in CryptoMinisat4's build directory and also on a system
with CryptoMinisat4 installed (usually <prefix>/lib/cmake/cryptominisat4/)