arch: Eliminate vtophys and its switching header file.

arch: Eliminate vtophys and its switching header file.

This function is no longer used anywhere in gem5.

Small helper functions which had been put alongside vtophys on ARM and
RISCV were also moved into src/arch/arm/remote_gdb.cc and
src/arch/power/pagetable.hh, the only places they were used.

Change-Id: Iba72f6c4b797a35a785a5bb781d602c943541fa7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26234
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>