vendor: Drop cmux

We removed our only consumer in the previous commit.  Generated with:

  $ go mod tidy
  $ go mod vendor
  $ git add -A pkg/cvo/metrics.go go.* vendor

using:

  $ go version
  go version go1.14.4 linux/arm64

go.mod

@@ -4,7 +4,6 @@ go 1.15
 
 require (
 	github.com/blang/semver/v4 v4.0.0
-	github.com/cockroachdb/cmux v0.0.0-20170110192607-30d10be49292
 	github.com/davecgh/go-spew v1.1.1
 	github.com/evanphx/json-patch v4.5.0+incompatible // indirect
 	github.com/ghodss/yaml v1.0.0

go.sum

@@ -65,8 +65,6 @@ github.com/chzyer/logex v1.1.10/go.mod h1:+Ywpsq7O8HXn0nuIou7OrIPyXbp3wmkHB+jjWR
 github.com/chzyer/readline v0.0.0-20180603132655-2972be24d48e/go.mod h1:nSuG5e5PlCu98SY8svDHJxuZscDgtXS6KTTbou5AhLI=
 github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1/go.mod h1:Q3SI9o4m/ZMnBNeIyt5eFwwo7qiLfzFZmjNmxjkiQlU=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
-github.com/cockroachdb/cmux v0.0.0-20170110192607-30d10be49292 h1:dzj1/xcivGjNPwwifh/dWTczkwcuqsXXFHY1X/TZMtw=
-github.com/cockroachdb/cmux v0.0.0-20170110192607-30d10be49292/go.mod h1:qRiX68mZX1lGBkTWyp3CLcenw9I94W2dLeRvMzcn9N4=
 github.com/cockroachdb/datadriven v0.0.0-20190809214429-80d97fb3cbaa/go.mod h1:zn76sxSg3SzpJ0PPJaLDCu+Bu0Lg3sKTORVIj19EIF8=
 github.com/containerd/continuity v0.0.0-20190827140505-75bee3e2ccb6/go.mod h1:GL3xCUCBDV3CZiTSEKksMWbLE66hEyuu9qyDOOqM47Y=
 github.com/coreos/bbolt v1.3.2/go.mod h1:iRUV2dpdMOn7Bo10OQBFzIJO9kkE559Wcmn+qkEiiKk=