mem-cache: Remove unnecessary block initialization on writeback

Change-Id: Ia9b825bcbb8d326705f74c15a93a88703153ba5a
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8283
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>