runtime: remove unused code in (*pallocBits).findLargeN

Change-Id: I9611edb9663a9c32bb23d26b9b22b79a54a97498
Reviewed-on: https://go-review.googlesource.com/c/go/+/586196
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mpallocbits.go

@@ -324,7 +324,6 @@ func (b *pallocBits) findLargeN(npages uintptr, searchIdx uint) (uint, uint) {
 		}
 		s := uint(sys.TrailingZeros64(x))
 		if s+size >= uint(npages) {
-			size += s
 			return start, newSearchIdx
 		}
 		if s < 64 {