rxrpc: Remove decl for rxrpc_kernel_call_is_complete()

rxrpc_kernel_call_is_complete() has been removed, so remove its declaration
too.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org