sunrpc: remove unused xpo_prep_reply_hdr callback

xpo_prep_reply_hdr are not used now.

It was defined for tcp transport only, however it cannot be
called indirectly, so let's move it to its caller and
remove unused callback.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>