[CMake] Fix typo in variable name. Whitespace cleanup.
[CMake] Get rid of DUNE_COMMON_FIELDVECTOR_SIZE_IS_METHOD which is already removed everywhere else.
[CMake] Fix smaller code quality issues, remove hard coded compiler flags.
[CMake] Remove check for __attribute__((always_inline)).

The test result was never used. The test slipped in as a copy from
the CMake code snippet from DuMuX.