MAINT remove unsupported documentation formats

In particular, "htmlhelp" and "qthelp". None of the core devs uses
this or is willing to support it. Fixes scikit-learn#2512 by not fixing it.

Author: larsmans

doc/Makefile

@@ -12,7 +12,7 @@ PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
 ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
-.PHONY: help clean html dirhtml pickle json htmlhelp qthelp latex latexpdf changes linkcheck doctest
+.PHONY: help clean html dirhtml pickle json latex latexpdf changes linkcheck doctest
 
 all: html-noplot
 
@@ -22,8 +22,6 @@ help:
 	@echo "  dirhtml   to make HTML files named index.html in directories"
 	@echo "  pickle    to make pickle files"
 	@echo "  json      to make JSON files"
-	@echo "  htmlhelp  to make HTML files and a HTML help project"
-	@echo "  qthelp    to make HTML files and a qthelp project"
 	@echo "  latex     to make LaTeX files, you can set PAPER=a4 or PAPER=letter"
 	@echo "  latexpdf   to make LaTeX files and run them through pdflatex"
 	@echo "  changes   to make an overview of all changed/added/deprecated items"
@@ -65,21 +63,6 @@ json:
 	@echo
 	@echo "Build finished; now you can process the JSON files."
 
-htmlhelp:
-	$(SPHINXBUILD) -b htmlhelp $(ALLSPHINXOPTS) $(BUILDDIR)/htmlhelp
-	@echo
-	@echo "Build finished; now you can run HTML Help Workshop with the" \
-	      ".hhp project file in $(BUILDDIR)/htmlhelp."
-
-qthelp:
-	$(SPHINXBUILD) -b qthelp $(ALLSPHINXOPTS) $(BUILDDIR)/qthelp
-	@echo
-	@echo "Build finished; now you can run "qcollectiongenerator" with the" \
-	      ".qhcp project file in $(BUILDDIR)/qthelp, like this:"
-	@echo "# qcollectiongenerator $(BUILDDIR)/qthelp/scikit-learn.qhcp"
-	@echo "To view the help file:"
-	@echo "# assistant -collectionFile $(BUILDDIR)/qthelp/scikit-learn.qhc"
-
 latex:
 	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
 	@echo