Suspend, test, and re-enable sharding + sharding_WT tasks on OS X builders

We're removing the sharding and sharding_WT tasks from OS X, OS X DEBUG and OS X SSL buildvariants temporarily while we're doing some work on the OS X builders themselves.

Initial commit on this issue will be to remove the tasks.

Once the OS X upgrades have been completed, someone needs to run a few patch builds to ensure that we've actually solved the problem with 6 hour runtimes and then re-enable these tasks.