FindCerf.cmake no longer needed

Package cerf provides a file cerfConfig.cmake [1]. Therefore package cerf can be found using CMake's "config mode" [2]. Therefore no more need to support "module mode" by providing module FindCerf.cmake. Therefore FindCerf.cmake should be removed from the project. The command find_package(Cerf MODULE REQUIRED) in CMakeLists.txt needs to be adapted.

[1] https://jugit.fz-juelich.de/mlz/libcerf/-/blob/main/cmake/cerfConfig.cmake.in [2] https://cmake.org/cmake/help/latest/command/find_package.html