arch-mips,arch-riscv,base: Get rid of the unused HexFile class.

arch-mips,arch-riscv,base: Get rid of the unused HexFile class.

A pointer to it was set up in the MIPS and RISCV system classes, but
nothing ever set that pointer. The class was put in base/loader, but
didn't have anything to do (as far as I can see) with loading anything
it had a loadSegments method, but was not a subclass of ObjectFile.

Change-Id: I4b711a31df20e20ffc306709227f60aa020fca15
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21464
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>