[fir] Remove obsolete fir.negf and fir.modf ops

fir.negf op is replaced by mlir.negf and
fir.modf is just deleted.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D110932



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>