Remove the ghc Makefile stuff for the old docbook xml

Not woth keeping it, none of it can sensibly be reused. The GHC
devs will need to add something new for the markdown user guide.

doc/Makefile

-TOP = ../../..
-
-ifeq "$(findstring boilerplate.mk, $(wildcard $(TOP)/mk/*))" ""
-# ----------------------------------------------------------------------------
-# Standalone Makefile:
-
-.PHONY: all
-
-all: Cabal.pdf
-
-Cabal.pdf: Cabal.xml
-	docbook2pdf Cabal.xml
-
-clean:
-	rm -fr *~ API users-guide Cabal.pdf Cabal.dvi semantic.cache
-
-else # boilerplate.mk exists
-# ----------------------------------------------------------------------------
-# GHC build tree Makefile:
-
-include $(TOP)/mk/boilerplate.mk
-
-DOC_SUBDIR=libraries/Cabal/doc
-XML_DOC = Cabal
-INSTALL_XML_DOC = $(XML_DOC)
-
-binary-dist:
-	@:
-
-include $(TOP)/mk/target.mk
-
-endif

doc/ghc.mk

-
-libraries/Cabal/doc_DOCBOOK_SOURCES := $(wildcard libraries/Cabal/doc/*.xml)
-
-$(eval $(call docbook,libraries/Cabal/doc,Cabal))
-