[Cleanup] Remove unsupported pre-processor directives

There are a lot of unsupported directives for cpp and fpp:

• pgiFortran: as mentionend in #121 (closed), we will drop support until the new flang compiler is available
• pgiFortran154: same as with pgiFortran
• GFORTRAN41: since we only support gcc >= 7.3 we can drop this one
• ABSOFT: we only support GNU, INTEL and NAG
• MPR_STANDALONE: MPR will be provided as a dependency starting with mHM v6.0 (@ottor should we keep it for now?)

I would like to remove these directives, so the code-base gets cleaner.