snapshot: Drop pointless function virDomainMomentIsCurrentName

snapshot: Drop pointless function virDomainMomentIsCurrentName

The qemu driver already had a full-blown virDomainMomentObjPtr to
check against, and the test driver ought to have one since we get
better error checking that the user passed in a valid object. Removes
the need for a helper function added in commit commit 4819f54b.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>