add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/erasure-decode_test.go

@@ -110,7 +110,7 @@ func TestErasureDecode(t *testing.T) {
 		for i, disk := range disks {
 			writers[i] = newBitrotWriter(disk, "testbucket", "object", erasure.ShardFileSize(test.data), writeAlgorithm, erasure.ShardSize())
 		}
-		n, err := erasure.Encode(context.Background(), bytes.NewReader(data[:]), writers, buffer, erasure.dataBlocks+1)
+		n, err := erasure.Encode(context.Background(), bytes.NewReader(data), writers, buffer, erasure.dataBlocks+1)
 		closeBitrotWriters(writers)
 		if err != nil {
 			setup.Remove()