fix typos in bout-config

[dschwoerer]

CONTRIBUTION.md hints that the use of Fortran is not directly preferred (i.e. to be never used).
bin/bout-config still provides a fortran mpi compiler :D

[d7919]

Whilst it's requested that there's no Fortran in BOUT++ I think this doesn't mean a Fortran project can't use the BOUT++ library (with appropriate wrapping), so the Fortran compiler flag could perhaps be useful for people who are trying to do this -- although I may be wrong!

[bendudson]

Sadly Fortran is sometimes needed, for example coupling to EIRENE. Whilst no Fortran should go in the core code, coupling to external libraries may be necessary.

[dschwoerer]

I am quite sure nobody used this.
Otherwise --fc would not return an error that it is an unknown option.

Further, $fc is set to none. Rather sure, you cannot compile EIRENE with this 👍

If I implement bout's configure to look for a fortran compiler, I can add this again, also providing apropriate flags ...