Revert "runtime: diagnose invalid pointers during GC"

Broke arm64. Update #9880.

This reverts commit 38d9b2a3.

Change-Id: I35fa21005af2183828a9d8b195ebcfbe45ec5138
Reviewed-on: https://go-review.googlesource.com/12247Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mbitmap.go

@@ -201,7 +201,7 @@ func heapBitsForObject(p uintptr) (base uintptr, hbits heapBits, s *mspan) {
 		// The following ensures that we are rigorous about what data
 		// structures hold valid pointers.
 		// TODO(rsc): Check if this still happens.
-		if true {
+		if false {
 			// Still happens sometimes. We don't know why.
 			printlock()
 			print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))