[Turbofan] Removing a minor TODO in the OSR part of the BytecodeGraphBuilder

The TODO was about wrapping together the sourceposition iterator and the bytecode
iterator. Since the first one is useful in fewer parts than the second, and the
bytecode iterator is more flexible to advance than the sourceposition iterator,
and we would not gain that much more readability, this TODO is removed.

TBR=mstarzinger@chromium.org

Bug: 
Change-Id: I104d0f5f0cd01686ea48d209419bd6bb2ed19bcf
Reviewed-on: https://chromium-review.googlesource.com/621106
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47432}

src/compiler/bytecode-graph-builder.cc

@@ -796,9 +796,6 @@ void BytecodeGraphBuilder::AdvanceToOsrEntryAndPeelLoops(
   }
 }
 
-// TODO(alexandret) create an iterator which wraps both bytecode_iterator and
-// source_position_iterator and increments them in one, so we don't need to
-// pass source_position_iterator as an argument here.
 void BytecodeGraphBuilder::VisitSingleBytecode(
     SourcePositionTableIterator* source_position_iterator) {
   const interpreter::BytecodeArrayIterator& iterator = bytecode_iterator();