cpu: Clear out some unnecessary ISA dependence in thread_context.hh.

The ISA version of the ISA class isn't used any more. Neither is
TheISA::MachInst.

Change-Id: I9085ad2b51ba19bf6e5bb17769dd048ac6384fec
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34821
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/thread_context.hh

@@ -58,7 +58,6 @@
 // DTB pointers.
 namespace TheISA
 {
-    class ISA;
     class Decoder;
 }
 class BaseCPU;
@@ -88,7 +87,6 @@ class System;
 class ThreadContext : public PCEventScope
 {
   protected:
-    typedef TheISA::MachInst MachInst;
     using VecRegContainer = TheISA::VecRegContainer;
     using VecElem = TheISA::VecElem;
     using VecPredRegContainer = TheISA::VecPredRegContainer;