Add staticcheck unused code check to CI

Makefile

@@ -263,6 +263,12 @@ gocyclo:
 	# Run gocyclo over all .go files
 	gocyclo -over 15 ${GOFILES}
 
+.PHONY: staticcheck
+staticcheck:
+	# check for unused code using staticcheck binary
+	# https://github.com/dominikh/go-tools/tree/master/cmd/staticcheck
+	staticcheck -tests=false -checks "U1000" ./agent/...
+
 # same as gofiles above, but without the `-f`
 .PHONY: govet
 govet:
@@ -276,7 +282,7 @@ importcheck:
 	@if [ -n "$(DIFFS)" ]; then echo "Files incorrectly formatted. Fix formatting by running goimports:"; echo "$(DIFFS)"; exit 1; fi
 
 .PHONY: static-check
-static-check: gocyclo govet importcheck
+static-check: gocyclo govet importcheck staticcheck
 
 .PHONY: goimports
 goimports:
@@ -287,6 +293,7 @@ goimports:
 	go get github.com/golang/mock/mockgen
 	go get golang.org/x/tools/cmd/goimports
 	go get github.com/fzipp/gocyclo
+	go get honnef.co/go/tools/cmd/staticcheck
 	touch .get-deps-stamp
 
 get-deps: .get-deps-stamp