[Dexter] Remove builder from Dexter

See "discussion":
  https://discourse.llvm.org/t/rfc-dexter-feature-removals/60462

This patch removes the builder functionality from Dexter, as it is an active
maintenance burden and is no longer required since Dexter is being invoked by
other test runners that can handle the build step better, and there has been no
objection that it is still needed.

Differential Revision: https://reviews.llvm.org/D151465