remove gh-pages Makefile target (not used)

docs/Makefile

@@ -130,14 +130,6 @@ doctest:
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
 
-gh-pages: html
-	GH_PAGES_CLONE="`mktemp -d`" && \
-	git clone --branch gh-pages `git rev-parse --show-toplevel` $$GH_PAGES_CLONE && \
-	(cd $$GH_PAGES_CLONE && git rm -r *) && \
-	cp -r _build/html/* $$GH_PAGES_CLONE && \
-	(cd $$GH_PAGES_CLONE && git add -A && git commit -m 'Updated gh-pages' && git push) && \
-	rm -rf $$GH_PAGES_CLONE
-
 gh-io: html
 	GH_IO_CLONE="`mktemp -d`" && \
     git clone git@github.com:borgbackup/borgbackup.github.io.git $$GH_IO_CLONE && \