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

src/internal/cm/hostlayout.go

@@ -1,5 +1,3 @@
-//go:build go1.23
-
 package cm
 
 import "structs"

src/internal/cm/hostlayout_go122.go

@@ -1,11 +0,0 @@
-//go:build !go1.23
-
-package cm
-
-// HostLayout marks a struct as using host memory layout.
-// See [structs.HostLayout] in Go 1.23 or later.
-type HostLayout struct {
-	_ hostLayout // prevent accidental conversion with plain struct{}
-}
-
-type hostLayout struct{}