runtime: remove TODO

I experimented with changing the write barrier to take the value in SI
rather than AX to improve register allocation. It had no effect on
performance and only made the "hello world" text 0.07% smaller, so
let's just remove the comment.

Change-Id: I6a261d14139b7a02a8467b31e74951dfb927ffb4
Reviewed-on: https://go-review.googlesource.com/78033Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/asm_amd64.s

@@ -2379,8 +2379,6 @@ TEXT runtime·addmoduledata(SB),NOSPLIT,$0-0
 // - AX is the value being written at DI
 // It clobbers FLAGS. It does not clobber any general-purpose registers,
 // but may clobber others (e.g., SSE registers).
-//
-// TODO: AX may be a bad choice because regalloc likes to use it.
 TEXT runtime·gcWriteBarrier(SB),NOSPLIT,$120
 	// Save the registers clobbered by the fast path.
 	//