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

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.

sum-of-multiples.md

@@ -5,15 +5,3 @@ The sum of these multiples is 78.
 
 Write a program that can find the sum of the multiples of a given set of
 numbers.
-
-This exercise is currently under revision.
-You are still free to do this exercise and submit solutions.
-Depending on whether this track has been updated recently,
-the tests may differ between this track and other tracks.
-
-In most tracks, the set of numbers must be explicitly provided.
-In some other tracks, if no set of numbers is given, it defaults to 3 and 5.
-We would like all tracks to require the set of numbers be explicit.
-
-If you would like to help out, please join us at
-https://github.com/exercism/x-common/issues/198.