[flang] Remove warnings that fast-math options are unimplemented

These are now fully implemented, see
https://reviews.llvm.org/D137390
https://reviews.llvm.org/D137391
https://reviews.llvm.org/D137456
https://reviews.llvm.org/D137580
https://reviews.llvm.org/D137602
https://reviews.llvm.org/D138048

These flags are still tested in
flang/test/Driver/frontend-forwarding.f90 and
flang/test/Lower/fast-math-arithmetic.f90

Differential revision: https://reviews.llvm.org/D138907