Remove DefaultSetup.hs

My investigation suggests it became unnecessary for the build with
revision 21dc55788a15166b00f3e653ebd70a22e27c9d1a (named
DefaultSetup.lhs at the time) but has hung around ever since.
Merge pull request #1180 from benmachine/springclean

Remove DefaultSetup.hs
Less verbose comments in cabal.sandbox.config.
Remove a spurious 'where'.
Don't add UserPackageDB to the sandbox PackageDBStack.

Fixes #1183.