KVM: selftests: always use manual clear in dirty_log_perf_test

Nothing sets USE_CLEAR_DIRTY_LOG anymore, so anything it surrounds
is dead code.

However, it is the recommended way to use the dirty page bitmap
for new enough kernel, so use it whenever KVM has the
KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 capability.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>