rpcrt4: Remove FIXME about RPC_CONTEXT_HANDLE_FLAGS from NDRSContextMarshall2 and NDRSContextUnmarshall2.

RPC_CONTEXT_HANDLE_FLAGS contain the values RPC_CONTEXT_HANDLE_SERIALIZE
or RPC_CONTEXT_HANDLE_DONT_SERIALIZE, however all the calls to underlying
binding->Assoc are guarded by a critical section, so it's redundant to
take any action about passed in Flags parameter.
Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>