golangci: bring back the unused linter (#13826)

https://github.com/golangci/golangci-lint/issues/885 is fixed wth 1.23.3.
We can now bring back unused.

build/Jenkinsfile.pr

@@ -103,7 +103,7 @@ pipeline {
 						sh """
 							cd /go/src/github.com/mattermost/mattermost-server
 							echo "Installing golangci-lint"
-							curl -sfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh| sh -s -- -b /usr/local/bin v1.23.0 # Keep this in sync with .golangci.yml and .circleci/config.yml
+							curl -sfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh| sh -s -- -b /usr/local/bin v1.23.3 # Keep this in sync with .golangci.yml and .circleci/config.yml
 							echo "Installing mattermost-govet"
 							GO111MODULE=off go get -u github.com/mattermost/mattermost-govet
 							export PATH=$PATH:$GOBIN