cpu: Remove the DecodeCache namespace

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

Change-Id: Ia1b2ab5444464f7c0ee85c8d288e38be4d7c013f
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67368
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/decode_cache.hh

@@ -38,7 +38,6 @@
 namespace gem5
 {
 
-GEM5_DEPRECATED_NAMESPACE(DecodeCache, decode_cache);
 namespace decode_cache
 {