Update console warning comment from pull-request-checklist.md when warning goes away

[liammulh]

From #171 (comment), @katestange noted that there is a comment in the PR checklist doc that says there's a warning about assets. We need to remove this comment when the warning goes away.

[gwhitney]

This should be resolved in #420.

[gwhitney]

Hmmm, actually we are up to 2 megabytes for the main chunk of the app, which is pretty huge. So I think this is out of scope for #420 after all, but it may well be related to #454 and so should get the same milestone as that, once a milestone is chosen for #454. Leaving this on alpha for now so we notice and deal with it once the milestone is set for #454.