lxd/migrate/instance: Don't ignore existing snapshot instances on migrate receiver
lxd/migrate/instance: Don't ignore existing snapshot instances on migrate receiver

When refreshing we should be deleting any snapshots that are due to be synced first,
so if there is already an instance snapshot record then something is awry.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd