misc: Remove lingering uses of TheISA::.

Change-Id: Ie55e0d79867fbc8f75a993fb456a58c84de5def4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62196
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/cpu/thread_context.hh

@@ -58,10 +58,6 @@ namespace gem5
 
 // @todo: Figure out a more architecture independent way to obtain the ITB and
 // DTB pointers.
-namespace TheISA
-{
-    class Decoder;
-}
 class BaseCPU;
 class BaseMMU;
 class BaseTLB;