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.

.circleci/config.yml

@@ -67,7 +67,7 @@ jobs:
       - run:
           command: |
             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 Jenkinsfile.pr
+            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 Jenkinsfile.pr
             echo "Installing mattermost-govet"
             export GOBIN=${PWD}/mattermost-server/bin
             GO111MODULE=off go get -u github.com/mattermost/mattermost-govet

.golangci.yml

@@ -1,5 +1,5 @@
 service:
-  golangci-lint-version: 1.23.0 # Keep this in sync with .circleci/config.yml and Jenkinsfile.pr
+  golangci-lint-version: 1.23.3 # Keep this in sync with .circleci/config.yml and Jenkinsfile.pr
 
 run:
   timeout: 5m
@@ -25,7 +25,7 @@ linters:
     - ineffassign
     - structcheck
     - unconvert
-    # - unused Disable until https://github.com/golangci/golangci-lint/issues/885 is fixed
+    - unused
     - varcheck
     # TODO: enable this later
     # - errcheck

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