compress/bzip2: remove dead code in huffman.go

The logic performs a series of shifts, which are useless given
that they are followed by an assignment that overrides the
value of the previous computation.

I suspect (but cannot prove) that this is leftover logic from an
original approach that attempted to store both the Huffman code
and the length within the same variable instead of using two
different variables as it currently does now.

Fixes #17949

Change-Id: Ibf6c807c6cef3b28bfdaf2b68d9bc13503ac21b2
Reviewed-on: https://go-review.googlesource.com/44091Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/compress/bzip2/huffman.go

@@ -108,10 +108,6 @@ func newHuffmanTree(lengths []uint8) (huffmanTree, error) {
 	codes := huffmanCodes(make([]huffmanCode, len(lengths)))
 	for i := len(pairs) - 1; i >= 0; i-- {
 		if length > pairs[i].length {
-			// If the code length decreases we shift in order to
-			// zero any bits beyond the end of the code.
-			length >>= 32 - pairs[i].length
-			length <<= 32 - pairs[i].length
 			length = pairs[i].length
 		}
 		codes[i].code = code