[Bug fortran/92463] Cleanups due to minimum MPFR version bump to 3.1.0

[jb at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92463

--- Comment #1 from Janne Blomqvist <jb at gcc dot gnu.org> ---
Seems r269139 can be reverted, that's apparently a patch to fix some accidental
breakage due to depending on a too new MPFR version.