[bigint] Remove obsolete TODO.

A while ago we introduced MutableBigInt in order to enforce this check.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I700ff0b1df854d4f6b8beff6f6c984e11cd07e40
Reviewed-on: https://chromium-review.googlesource.com/881174Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50823}

src/objects-debug.cc

@@ -1256,8 +1256,6 @@ void BigInt::BigIntVerify() {
   CHECK(IsBigInt());
   CHECK_GE(length(), 0);
   CHECK_IMPLIES(is_zero(), !sign());  // There is no -0n.
-  // TODO(neis): Somewhere check that MSD is non-zero. Doesn't hold during some
-  // operations that allocate which is why we can't test it here.
 }
 
 void JSModuleNamespace::JSModuleNamespaceVerify() {