[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
Bug 92463 depends on bug 91828, which changed state.

Bug 91828 Summary: gcc/fortran/check.c requires mpfr_set_z_2exp from MPFR 
3.0.0, unavailable in mpfr-2.4.2
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91828

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED