chore: Don't print which error in Makefile (#1012)

The which binary is used to detect if client-gen is installed, and if
it's not, the Makefile will install it. The initial detection prints
an error if it's not found. This is misleading, as it is actually an
expected situation.

Author: twz123

Makefile

@@ -130,7 +130,7 @@ CONTROLLER_GEN=$(shell which controller-gen)
 
 .PHONY: client-gen
 client-gen:
-ifeq (, $(shell which client-gen))
+ifeq (, $(shell which client-gen 2>/dev/null))
 	go install k8s.io/code-generator/cmd/[email protected]
 CLIENT_GEN=$(shell go env GOPATH)/bin/client-gen
 else