Drop obsolete libffi Makefile

This patch drops obsolete libffi Makefile from the tree, given it's
completely unused since removal of make build system in !7094.

(cherry picked from commit 3fe843c7)
(cherry picked from commit 81ddb1e7)
(cherry picked from commit cde02d8d)

libffi/Makefile

-# -----------------------------------------------------------------------------
-#
-# (c) 2009 The University of Glasgow
-#
-# This file is part of the GHC build system.
-#
-# To understand how the build system works and how to modify it, see
-#      https://gitlab.haskell.org/ghc/ghc/wikis/building/architecture
-#      https://gitlab.haskell.org/ghc/ghc/wikis/building/modifying
-#
-# -----------------------------------------------------------------------------
-
-dir = libffi
-TOP = ..
-include $(TOP)/mk/sub-makefile.mk

libffi/ln

-#!/bin/sh
-exit 1
-