SUNRPC: Remove stale comments

bc_close() and bc_destroy now do something, so the comments are
no longer correct. Commit 6221f1d9 ("SUNRPC: Fix backchannel
RPC soft lockups") should have removed these.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>