socket: remove tautological condition in so_unsplice()

[kevans]

Summary

so2rele was introduced in 1000cc4a0d3 and it was necessary there, but
the cleanup in a837d1fe49e0255 rendered it redundant if our own KASSERT
is to be believed: we've asserted that so2->so_splice_back == sp and
sp has been dereferenced above, so there's no condition left where
we shouldn't release the socket reference at the end. Indeed, the
change in so_splice() to NULL out sp->dst removes that possible state of
a partially constructed splice: if sp->dst is set, it has been ref'd.