Reduce memory allocation

[bpot]

This reverts commit fc1278e.

It appears that using a buffer here is no longer necessary after the
upstream fix gogo/protobuf#504

The bufio.NewWriter looks to be responsible for nearly half the allocations (by size) on a heavily loaded ipfs daemon:

(pprof) peek msgToStream
Showing nodes accounting for 193647.89MB, 100% of 193647.89MB total
----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context 	 	 
----------------------------------------------------------+-------------
                                        92608.33MB   100% |   gx/ipfs/QmcSPuzpSbVLU6UHU4e5PwZpm4fHbCn5SbNR5ZNL6Mj63G/go-bitswap/network.(*streamMessageSender).SendMsg
                                            7.47MB 0.0081% |   gx/ipfs/QmcSPuzpSbVLU6UHU4e5PwZpm4fHbCn5SbNR5ZNL6Mj63G/go-bitswap/network.(*impl).SendMessage
         0     0%     0% 92615.80MB 47.83%                | gx/ipfs/QmcSPuzpSbVLU6UHU4e5PwZpm4fHbCn5SbNR5ZNL6Mj63G/go-bitswap/network.msgToStream
                                        80303.55MB 86.71% |   bufio.NewWriter
                                         8285.02MB  8.95% |   gx/ipfs/QmcSPuzpSbVLU6UHU4e5PwZpm4fHbCn5SbNR5ZNL6Mj63G/go-bitswap/message.(*impl).ToNetV1
                                         2802.69MB  3.03% |   bufio.(*Writer).Flush
                                         1216.04MB  1.31% |   gx/ipfs/Qma3Xp3FXFSP4prirEiRYHJ2tgGE8EAx9i6JLziPLpAQjq/go-libp2p-swarm.(*Stream).SetWriteDeadline
                                            8.50MB 0.0092% |   gx/ipfs/QmcSPuzpSbVLU6UHU4e5PwZpm4fHbCn5SbNR5ZNL6Mj63G/go-bitswap/message.(*impl).ToNetV0

[Stebalien]

Thanks!