We have decided not to backport the various improvements we are making to stabilize/polish the slot-based execution engine to the 5.0 branch. This means that we will be unable to release SBE to customers in a future 5.0.x release. Since the "SBE on" configuration will be completely unsupported in 5.0, we should stop testing it. To be more precise, we should delete the handful of build variants defined in evergreen.yml which test the server with SBE enabled. This change should target only the 5.0 branch; we will continue to test and develop SBE in the master branch.

This change is important to avoid distracting the team with build failures from the 5.0 branch which need not be acted upon.