cmd/compile: remove outdate TODO in inl.go

Mid-stack inlining is enable now, see #19348, but we still can not
remove the special case for runtime.heapBits.nextArena, because
runtime.heapBits.next is too complex to be inlined
(cost 96 exceeds budget 80).

Change-Id: I04ea86509074afdc83a3f70d68b8a1a8829763d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/174839Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/inl.go

@@ -343,10 +343,6 @@ func (v *hairyVisitor) visit(n *Node) bool {
 				// runtime.heapBits.next even though
 				// it calls slow-path
 				// runtime.heapBits.nextArena.
-				//
-				// TODO(austin): Once mid-stack
-				// inlining is the default, remove
-				// this special case.
 				break
 			}
 		}