sum-of-multiples: Mark as no longer under revision

[petertseng]

This partially reverts 1f0ad90,
which partially reverted 89308fd.

The default behavior was added in #157 as a result of confusion in
exercism/exercism#2654. Since then, we have
decided the way forward is to remove the default behavior altogether, as
discussed in #198.

This commit should be held until all tracks have removed their
expectations of default behavior.

[petertseng]

Hello, it's been a month. I think that's been quite ample notice. In a few hours, I would like to:

• Submit PRs to the remaining tracks myself
• Not wait for them to merge those PRs, instead just merge this PR when I submit those

[petertseng]

Updated plan: Instead, modify this PR to make the README say "This exercise is currently undergoing change, some tracks have defaults, some don't, if you want to help look at #198", then I'll merge.

[petertseng]

That was done in #236. So the above plan now should read:

• Make a new PR to state that this exercise is currently undergoing change (sum-of-multiples: Note change in flight in README #236). Merge that PR ASAP (done).
• Continue to wait on this PR until all tracks have completed the change. See progress above in linked issues.

[IanWhitney]

@petertseng If I'm reading your comments correctly, we can merge this. Yes?

[petertseng]

No, I should have clarified. What was done was that #236 was merged. The comment that says "then I'll merge" applied to the PR that says "this exercise is undergoing change", which was #236.

This PR here should only be merged after all tracks complete - currently clojure, cpp, and lfe have yet to do