Fix make generate

In #80
we did a hack to ensure code was generated even if it wasn't
placed at $GOPATH sigs.k8s.io/network-policy-api.

But that's not totally necessary always, like in my case where it
was in that place, with this new change it started wiping off my
entire sigs.k8s.io folder locally when I run make generate.

Instead let's make generate-setup and generate-cleanup optional
as separate make commands and not club them with make generate.
These kind of kinks depend on how each user's local setup is.

Signed-off-by: Surya Seetharaman <[email protected]>

Author: tssurya

Makefile

@@ -47,7 +47,7 @@ vet: ## Run go vet against code.
 	go vet ./...
 
 .PHONY: generate
-generate: generate-setup generate-deepcopy generate-typed-clients generate-typed-listers generate-typed-informers generate-cleanup
+generate: generate-deepcopy generate-typed-clients generate-typed-listers generate-typed-informers
 
 .PHONY: generate-setup
 generate-setup: