Remove make target for web app for GSA#783 post-GSA#841

It turns out I left an errant Makefile target that is no longer needed.
See the removed target from the targeted Makefile module, but it was
left in the top-level targets and referenced in the still active `clean`
target.

https://github.com/GSA/fedramp-automation/pull/841/files#diff-b0fa8ca7ead097f81107867d3e7cea11f7b622267b326985e8d7f8763031ea1a

Makefile

@@ -19,7 +19,7 @@ configure: init-validations
 init-repo:
 	git submodule update --init --recursive
 
-clean: clean-dist clean-validations clean-web  ## Clean all
+clean: clean-dist clean-validations ## Clean all
 
 clean-dist:  ## Clean non-RCS-tracked dist files
 	@echo "Cleaning dist..."