KVM: Remove unused config symbol

The dirty patch logging series introduced both
HAVE_KVM_ARCH_DIRTY_LOG_PROTECT and KVM_GENERIC_DIRTYLOG_READ_PROTECT
config symbols, but only KVM_GENERIC_DIRTYLOG_READ_PROTECT is used.
Just remove the unused one.

(The config symbol was renamed during the development of the patch
series and the old name just creeped in by accident.()

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>