Avoid unnecessary Fortran checks

VELOCIraptor doesn't use Fortran, and therefore checking for a Fortran
compiler and runtime libraries is not necessary. Furthermore this can
introduce unnecessary problems; in my case it prevented me to compile
SWIFT with VELOCIraptor in our local platform.

I suspect the appearance of these checks is a remnant from a potential
copy-paste from the grackle integration checks appearing just before
VELOCIraptor's.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>