Address a TODO about the need for DecompressedSize API

[easwars]

With the switch to the new encoding API and the mem package, we have a TODO and some commented out code here: https://github.com/grpc/grpc-go/blob/master/rpc_util.go#L880-L900

It looks like there are no current usages of DecompressedSize defined here:

grpc-go/encoding/encoding.go

Line 42 in ada6787

// If a Compressor implements `DecompressedSize(compressedBytes []byte) int`,

There is a definition of this method here:

grpc-go/encoding/gzip/gzip.go

Line 117 in ada6787

func (c *compressor) DecompressedSize(buf []byte) int {

It looks to me that we could get rid of the commented out code in rpc_util.go and get rid of the docstring that says gRPC will use this if present.

@dfawley @PapaCharlie