Remove RAWCPP_FLAGS

Summary:
#9094 mentions to "remove the RAW_CPP bits from the ghc build system because
they're not longer needed", "once the CPP settings ticket is merged #8683"

#8683 was merged with 34f7e9a3, Phab:D26.

Test Plan: harbormaster

Reviewers: carter, austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D240

GHC Trac Issues: #9094