Remove build-depends from pkgDescrFieldDescrs

Remove the build-depends field from the top-level parser of Cabal files.

Why do I think this is OK? Two reasons:

1. We never consult buildDepends until after `Distribution/PackageDescription/Configuration.hs` runs, in which case the contents of the field get overwritten with the true dependency picks. We do consult the field for some package syntax checks but as I argue in https://github.com/haskell/cabal/issues/2065 this the wrong place to try to do the check.

2. The `build-depends` field already gets lifted up by special case code in `sectionizeFields` to propagate it to the stanzas, which is the *true* codepath how we account for these build-depends.

Duncan would feel better if we ran this modification on Hackage to make sure it's good. But based on code reading and light testing, I think it's right.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>