rlp/rlpgen: remove build tag (#28106)

* rlp/rlpgen: remove build tag

This tag was supposed to prevent unstable output when types reference each other. Imagine
there are two struct types A and B, where a reference to type B is in A. If I run rlpgen
on type B first, and then on type A, the generator will see the B.EncodeRLP method and
call it. However, if I run rlpgen on type A first, it will inline the encoding of B.

The solution I chose for the initial release of rlpgen was to just ignore methods
generated by rlpgen using a build tag. But there is a problem with this: if any code in
the package calls EncodeRLP explicitly, the package can't be loaded without errors anymore
in rlpgen, because the loader ignores it. Would be nice if there was a way to just make it
ignore invalid functions during type checking (they're not necessary for rlpgen), but
golang.org/x/tools/go/packages does not provide a way of ignoring them.

Luckily, the types we use rlpgen with do not reference each other right now, so we can
just remove the build tags for now.

rlp/rlpgen/main.go

@@ -73,9 +73,8 @@ type Config struct {
 func (cfg *Config) process() (code []byte, err error) {
 	// Load packages.
 	pcfg := &packages.Config{
-		Mode:       packages.NeedName | packages.NeedTypes | packages.NeedImports | packages.NeedDeps,
-		Dir:        cfg.Dir,
-		BuildFlags: []string{"-tags", "norlpgen"},
+		Mode: packages.NeedName | packages.NeedTypes,
+		Dir:  cfg.Dir,
 	}
 	ps, err := packages.Load(pcfg, pathOfPackageRLP, ".")
 	if err != nil {
@@ -117,8 +116,6 @@ func (cfg *Config) process() (code []byte, err error) {
 	// This is done here to avoid processing these lines with gofmt.
 	var header bytes.Buffer
 	fmt.Fprint(&header, "// Code generated by rlpgen. DO NOT EDIT.\n\n")
-	fmt.Fprint(&header, "//go:build !norlpgen\n")
-	fmt.Fprint(&header, "// +build !norlpgen\n\n")
 	return append(header.Bytes(), code...), nil
 }