all: clean up code with Go 1.24+ in mind (#5489)

* all: clean up code with Go 1.24+ in mind

* all: more old go cleanup

* all: even remove rand_fastrand64

* runtime: revert rand changes

* runtime: re-drop rand_fastrand64

cgo/cgo.go

@@ -26,10 +26,6 @@ import (
 	"golang.org/x/tools/go/ast/astutil"
 )
 
-// Function that's only defined in Go 1.22.
-var setASTFileFields = func(f *ast.File, start, end token.Pos) {
-}
-
 // cgoPackage holds all CGo-related information of a package.
 type cgoPackage struct {
 	generated       *ast.File

cgo/cgo_go122.go

@@ -1,17 +0,0 @@
-//go:build go1.22
-
-package cgo
-
-// Code specifically for Go 1.22.
-
-import (
-	"go/ast"
-	"go/token"
-)
-
-func init() {
-	setASTFileFields = func(f *ast.File, start, end token.Pos) {
-		f.FileStart = start
-		f.FileEnd = end
-	}
-}

cgo/libclang.go

@@ -592,7 +592,8 @@ func (p *cgoPackage) getClangLocationPosition(location C.CXSourceLocation, tu C.
 			Package: f.Pos(0),
 			Name:    ast.NewIdent(p.packageName),
 		}
-		setASTFileFields(astFile, f.Pos(0), f.Pos(int(size)))
+		astFile.FileStart = f.Pos(0)
+		astFile.FileEnd = f.Pos(int(size))
 		p.cgoFiles = append(p.cgoFiles, astFile)
 	}
 	positionFile := p.tokenFiles[filename]