internal/goexperiment: delete CacheProg goexperiment

It is now always enabeld. The GOEXPERIMENT doesn't control
anything. Remove.

Change-Id: I24ecf0cd7be5d5dd20f4c558871a3ea97792055e
Reviewed-on: https://go-review.googlesource.com/c/go/+/684837
Reviewed-by: qiu laidongfeng <2645477756@qq.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: cherrymui

src/internal/goexperiment/exp_cacheprog_off.go

@@ -96,10 +96,6 @@ type Flags struct {
 	// copy of the iteration variable.
 	LoopVar bool
 
-	// CacheProg adds support to cmd/go to use a child process to implement
-	// the build cache; see https://github.com/golang/go/issues/59719.
-	CacheProg bool
-
 	// NewInliner enables a new+improved version of the function
 	// inlining phase within the Go compiler.
 	NewInliner bool