MINOR: stream-int: remove dependency on CO_FL_WAIT_ROOM for rcv_buf()

MINOR: stream-int: remove dependency on CO_FL_WAIT_ROOM for rcv_buf()

The only case where this made sense was with mux_h1 but Since we
introduced CS_FL_MAY_SPLICE, we don't need to rely on this anymore,
thus we don't need to clear it either when we do not splice.

There is a last check on this flag used to determine if the rx channel
is full and that cannot go away unless it's changed to use the CS
instead but for now this wouldn't add any benefit so better not do
it yet.