Prune dependencies (#1917)

When running `dep prune` explicitly, the following message show up:
```
dep prune
Pruning is now performed automatically by dep ensure.
```

However, after the explicit `dep prune`, there are still many files deleted. (Guess `dep ensure` is not complete yet).

This fix did a `dep prune` to clean up unneeded files.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

vendor/google.golang.org/grpc/encoding/gzip/gzip.go

@@ -1,117 +0,0 @@
-/*
- *
- * Copyright 2017 gRPC authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- *
- */
-
-// Package gzip implements and registers the gzip compressor
-// during the initialization.
-// This package is EXPERIMENTAL.
-package gzip
-
-import (
-	"compress/gzip"
-	"fmt"
-	"io"
-	"io/ioutil"
-	"sync"
-
-	"google.golang.org/grpc/encoding"
-)
-
-// Name is the name registered for the gzip compressor.
-const Name = "gzip"
-
-func init() {
-	c := &compressor{}
-	c.poolCompressor.New = func() interface{} {
-		return &writer{Writer: gzip.NewWriter(ioutil.Discard), pool: &c.poolCompressor}
-	}
-	encoding.RegisterCompressor(c)
-}
-
-type writer struct {
-	*gzip.Writer
-	pool *sync.Pool
-}
-
-// SetLevel updates the registered gzip compressor to use the compression level specified (gzip.HuffmanOnly is not supported).
-// NOTE: this function must only be called during initialization time (i.e. in an init() function),
-// and is not thread-safe.
-//
-// The error returned will be nil if the specified level is valid.
-func SetLevel(level int) error {
-	if level < gzip.DefaultCompression || level > gzip.BestCompression {
-		return fmt.Errorf("grpc: invalid gzip compression level: %d", level)
-	}
-	c := encoding.GetCompressor(Name).(*compressor)
-	c.poolCompressor.New = func() interface{} {
-		w, err := gzip.NewWriterLevel(ioutil.Discard, level)
-		if err != nil {
-			panic(err)
-		}
-		return &writer{Writer: w, pool: &c.poolCompressor}
-	}
-	return nil
-}
-
-func (c *compressor) Compress(w io.Writer) (io.WriteCloser, error) {
-	z := c.poolCompressor.Get().(*writer)
-	z.Writer.Reset(w)
-	return z, nil
-}
-
-func (z *writer) Close() error {
-	defer z.pool.Put(z)
-	return z.Writer.Close()
-}
-
-type reader struct {
-	*gzip.Reader
-	pool *sync.Pool
-}
-
-func (c *compressor) Decompress(r io.Reader) (io.Reader, error) {
-	z, inPool := c.poolDecompressor.Get().(*reader)
-	if !inPool {
-		newZ, err := gzip.NewReader(r)
-		if err != nil {
-			return nil, err
-		}
-		return &reader{Reader: newZ, pool: &c.poolDecompressor}, nil
-	}
-	if err := z.Reset(r); err != nil {
-		c.poolDecompressor.Put(z)
-		return nil, err
-	}
-	return z, nil
-}
-
-func (z *reader) Read(p []byte) (n int, err error) {
-	n, err = z.Reader.Read(p)
-	if err == io.EOF {
-		z.pool.Put(z)
-	}
-	return n, err
-}
-
-func (c *compressor) Name() string {
-	return Name
-}
-
-type compressor struct {
-	poolCompressor   sync.Pool
-	poolDecompressor sync.Pool
-}