runtime: remove markBits.clearMarkedNonAtomic

It's not used, it's never been used, and it doesn't do what its doc
comment says it does.

Fixes #18941.

Change-Id: Ia89d97fb87525f5b861d7701f919e0d6b7cbd376
Reviewed-on: https://go-review.googlesource.com/36322Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mbitmap.go

@@ -332,11 +332,6 @@ func (m markBits) clearMarked() {
 	atomic.And8(m.bytep, ^m.mask)
 }
 
-// clearMarkedNonAtomic clears the marked bit non-atomically.
-func (m markBits) clearMarkedNonAtomic() {
-	*m.bytep ^= m.mask
-}
-
 // markBitsForSpan returns the markBits for the span base address base.
 func markBitsForSpan(base uintptr) (mbits markBits) {
 	if base < mheap_.arena_start || base >= mheap_.arena_used {