[wasm] Remove impossible TODO

std::vector can never store const types, as the stored type has to be
either copy-constructable or move-constructable.
std::vector<const X> does not compile and makes no sense if you think
about it.
Thus remove the TODO to use such a vector.

R=ahaas@chromium.org

Change-Id: Ieb00a31872f04c720d2ef90b70452c18e79f0a5d
Reviewed-on: https://chromium-review.googlesource.com/503148
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45260}

src/wasm/wasm-interpreter.cc

@@ -698,7 +698,6 @@ class SideTable : public ZoneObject {
       uint32_t target_stack_height;
       // Arity when branching to this label.
       const uint32_t arity;
-      // TODO(clemensh): Fix ZoneAllocator and make this ZoneVector<const Ref>.
       ZoneVector<Ref> refs;
 
       static CLabel* New(Zone* zone, uint32_t stack_height, uint32_t arity) {