Remove dll-split.

This patch removes dll-split from the code base, the reason is dll-split
no longer makes any sense. It was designed to split a dll in two, but we
now already have many more symbols than would fit inside two dlls. So we
need a third one. This means there's no point in having to maintain this
list as it'll never work anyway and the solution isn't scalable.

Test Plan: ./validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, #ghc_windows_task_force

GHC Trac Issues: #5987

Differential Revision: https://phabricator.haskell.org/D3882