Add init and clean rules to Makefile, remove install

init checks out straight submodule, clean removes straight build and
repos directories, then checks out submodule again (since it's also
stored in repos directory)

Makefile

@@ -1,6 +1,12 @@
-.PHONY: default install
+.PHONY: default init clean
 
-default: install
+default: init
 
-install:
-	emacs --script install.el --kill
+init:
+	git submodule update --init straight/repos/straight.el/
+
+clean:
+	-rm straight/build-cache.el
+	-rm -rf straight/build
+	-rm -rf straight/repos
+	make init