Removed IntelMPI Workaround from CMake

- workaround seems to be not necessary any more, for recent IntelMPI versions
- the workaround did not play with CMake's try_compile function, which tries to compile some test programs to check for compiler features
  The workaround added '-include mpi.h' to the compile flags, which are also used by try_compile, but try_compile does not set include directories with "-I"
  This lead to the problem, that for example the OpenMP flag was not correctly determined, since there the try_compile mechanism is used