* Removed the webapp-secure build flag, rolling it into the webapp build
  flag.
* Removed the quvi and tahoe build flags, which only adds aeson to
  the core dependencies.
* Removed the feed build flag, which only adds feed to the core
  dependencies.

Build flags have cost in both code complexity and also make Setup configure
have to work harder to find a usable set of build flags when some
dependencies are missing.