Revert "Reapply "[Dexter] Remove builder from Dexter""

Re-application of the Dexter builder removal reversed due to continued
errors on the green dragon LLDB buildbot:
https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/59716/

Cause of the error is unclear, but it looks as though there is some
unexpected non-determinism in the test failures.

This reverts commit 32327045.