Revert "Update Haddock submodule to latest `master` tip"

This reverts commit 7f634320a2c39f4f81f631deb844acbaebaced66 again for now as
it causes validate's bindist phase to fail with

  haddock: internal error: .../install   dir/lib/ghc-7.9.20141214/html:
     getDirectoryContents: does not exist (No such file or directory)