encoding: remove unused DecompressedSize API (grpc#8830)

Fixes: grpc#7788 

This change removes the unused `DecompressedSize` API from the gzip
`compressor` and the corresponding documentation in the encoding
package.

RELEASE NOTES: N/A

Author: Pranjali-2501

encoding/encoding.go

@@ -58,10 +58,6 @@ func init() {
 
 // Compressor is used for compressing and decompressing when sending or
 // receiving messages.
-//
-// If a Compressor implements `DecompressedSize(compressedBytes []byte) int`,
-// gRPC will invoke it to determine the size of the buffer allocated for the
-// result of decompression.  A return value of -1 indicates unknown size.
 type Compressor interface {
 	// Compress writes the data written to wc to w after compressing it.  If an
 	// error occurs while initializing the compressor, that error is returned

encoding/gzip/gzip.go

@@ -27,7 +27,6 @@ package gzip
 
 import (
 	"compress/gzip"
-	"encoding/binary"
 	"fmt"
 	"io"
 	"sync"
@@ -111,17 +110,6 @@ func (z *reader) Read(p []byte) (n int, err error) {
 	return n, err
 }
 
-// RFC1952 specifies that the last four bytes "contains the size of
-// the original (uncompressed) input data modulo 2^32."
-// gRPC has a max message size of 2GB so we don't need to worry about wraparound.
-func (c *compressor) DecompressedSize(buf []byte) int {
-	last := len(buf)
-	if last < 4 {
-		return -1
-	}
-	return int(binary.LittleEndian.Uint32(buf[last-4 : last]))
-}
-
 func (c *compressor) Name() string {
 	return Name
 }