Partially revert "add target html and gallinahtml"

This partially reverts 2bf11f4f.  There is no need to add new Makefile targets as the Makefile already forwards all targets to Makefile.coq.

Makefile

@@ -36,12 +36,6 @@ build-dep:
 	opam install opam-builddep-temp --deps-only $(YFLAG)
 	opam pin remove opam-builddep-temp
 
-html: Makefile.coq
-	+@make -f Makefile.coq html
-
-gallinahtml: Makefile.coq
-	+@make -f Makefile.coq gallinahtml
-
 # Some files that do *not* need to be forwarded to Makefile.coq
 Makefile: ;
 _CoqProject: ;