cmd/compile: remove CLOBBERDEAD experiment

This experiment is less effective and less needed since the
introduction of stack objects.

We can't clobber stack objects because we don't know statically
whether they are live or not.

We don't really need this experiment that much any more, as it was
primarily used to test the complicated ambiguously-live logic in the
liveness analysis, which has been removed in favor of stack objects.

It is also ~infeasible to maintain once we have safepoints everywhere.

Fixes #27326

Change-Id: I3bdde480b93dd508d048703055d4586b496176af
Reviewed-on: https://go-review.googlesource.com/c/151317
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/objabi/util.go

@@ -104,7 +104,6 @@ var (
 	framepointer_enabled     int = 1
 	Fieldtrack_enabled       int
 	Preemptibleloops_enabled int
-	Clobberdead_enabled      int
 )
 
 // Toolchain experiments.
@@ -118,7 +117,6 @@ var exper = []struct {
 	{"fieldtrack", &Fieldtrack_enabled},
 	{"framepointer", &framepointer_enabled},
 	{"preemptibleloops", &Preemptibleloops_enabled},
-	{"clobberdead", &Clobberdead_enabled},
 }
 
 var defaultExpstring = Expstring()