gometalinter doesn't have unused

[teawater]

kata-containers/agent#493

INFO: Forcing gometalinter version 
Installing:
  deadcode
  dupl
  errcheck
  gochecknoglobals
  gochecknoinits
  goconst
  gocyclo
  goimports
  golint
  gosec
  gotype
  gotypex
  ineffassign
  interfacer
  lll
  maligned
  misspell
  nakedret
  safesql
  staticcheck
  structcheck
  unconvert
  unparam
  varcheck
INFO: gometalinter args: '--exclude=vendor/.*" --tests --exclude=".*\.pb\.go" --disable-all --enable=gofmt --concurrency=2 --enable=misspell --enable=vet --enable=ineffassign --enable=gocyclo --cyclo-over=15 --enable=golint --deadline=600s --enable=structcheck --enable=unused --enable=staticcheck --enable=maligned --enable=varcheck --enable=unconvert'

gometalinter: error: unknown linters: unused
The command ".ci/static-checks.sh" failed and exited with 1 during .

[teawater]

I think we can just wait for #1315 to fix this issue.