sim,arch: Remove the GuestABI namespace

This namespace has gone through the deprecation period
and can now be removed.

Change-Id: I476815491314f4222da43da75c91654b4f3d1228
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67374
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/proxy_ptr.test.cc

@@ -474,7 +474,6 @@ struct TestABI
 namespace gem5
 {
 
-GEM5_DEPRECATED_NAMESPACE(GuestABI, guest_abi);
 namespace guest_abi
 {