mem-cache: Remove the ReplacementPolicy namespace

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

Change-Id: If4904706b897999e9200b163d47679519f01e4d4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67352
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/cache/prefetch/stride.hh

@@ -64,7 +64,6 @@ namespace gem5
 {
 
 class BaseIndexingPolicy;
-GEM5_DEPRECATED_NAMESPACE(ReplacementPolicy, replacement_policy);
 namespace replacement_policy
 {
     class Base;