internal/impl: CompressGZIP: simplify
internal/impl: CompressGZIP: simplify

This simplifies the code and fixes the following linter warning
reported by staticcheck:

        internal/impl/legacy_export.go:84:51: uint16(blockSize) ^ 0x0000 always equals uint16(blockSize) (SA4016)

Similar code:
 https://cs.opensource.google/go/go/+/master:src/compress/flate/huffman_bit_writer.go;l=413-414;drc=19309779ac5e2f5a2fd3cbb34421dafb2855ac21

Change-Id: Id099b0de635f668d44f57bacd89b564337560254
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/406841
Reviewed-by: Cassondra Foesch <e5198b7ece30e41e2a270b9f943dd90e812051ff@gmail.com>
Reviewed-by: Lasse Folger <c1a9282e9a97d9bcfa578c9e6643dbf90dcebfeb@google.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go