Remove `SLWaitUntilTrue`.

[wearhere]

In the 2.0 timeframe, replaced by SLIsTrueWithTimeout.

SLWaitUntilTrue will have been deprecated in the 1.2 timeframe: #241.

[wearhere]

This should be tackled at the very end of the 2.0 release cycle in case we need/want to roll another 1.x release.