Fix the regression caused by f31c630c8cc15c4de1cc7e45b6c5c8053d5bcb75

f31c630c8cc15c4de1cc7e45b6c5c8053d5bcb75 was causing
SocketGatheringWriteTest to fail because it does not take the case where
an empty buffer exists in a gathering write.

When there is an empty buffer in a gathering write, the number of
buffers returned by ChannelOutboundBuffer.nioBuffer() and the actual
number of write attemps can differ.

To remove the write requests correctly, a byte transport must use
ChannelOutboundBuffer.removeBytes()