[gtk-doc] Remove hack for deleting tmpl/

This was introduced in 029f46bd
and was in fact a bug in gtk-doc master:

  https://bugzilla.gnome.org/show_bug.cgi?id=707426

docs/reference/Makefile.am

@@ -109,9 +109,4 @@ TESTS_ENVIRONMENT = cd $(srcdir) && \
 #TESTS = $(GTKDOC_CHECK)
 endif
 
-maintainer-clean-local: maintainer-clean-tmpl
-
-maintainer-clean-tmpl:
-	-rm -rf $(srcdir)/tmpl
-
 -include $(top_srcdir)/git.mk