nfsd4: remove unnecessary comment

For the most part readers of cl_cb_state only need a value that is
"eventually" right.  And the value is set only either 1) in response to
some change of state, in which case it's set to UNKNOWN and then a
callback rpc is sent to probe the real state, or b) in the handling of a
response to such a callback.  UNKNOWN is therefore always a "temporary"
state, and for the other states we're happy to accept last writer wins.

So I think we're OK here.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>