runtime: drop haveexperiment, sys.GOEXPERIMENT

We have ways to statically access experiments now, so we don't need a
relatively clunky string-parsing dynamic way to do it.

Change-Id: I5d75480916eef4bde2c30d5fe30593180da77ff2
Reviewed-on: https://go-review.googlesource.com/c/go/+/307815
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/internal/objabi/util.go

@@ -171,7 +171,6 @@ func init() {
 	}
 
 	// Set GOEXPERIMENT to the parsed and canonicalized set of experiments.
-	// This format must be parseable by runtime.haveexperiment.
 	GOEXPERIMENT = expList()
 }