remove unused dependency (#661)

Remove unused dependency in `oauth2` module.

### Verifying this change

- [x] Make sure that the change passes the CI checks.


This change is a trivial rework / code cleanup without any test coverage.

oauth2/go.mod

@@ -4,7 +4,6 @@ go 1.13
 
 require (
 	github.com/99designs/keyring v1.1.6
-	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/form3tech-oss/jwt-go v3.2.3+incompatible
 	github.com/onsi/ginkgo v1.14.0
 	github.com/onsi/gomega v1.10.1

oauth2/go.sum

@@ -6,8 +6,6 @@ github.com/danieljoos/wincred v1.0.2/go.mod h1:SnuYRW9lp1oJrZX/dXJqr0cPK5gYXqx3E
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/dgrijalva/jwt-go v3.2.0+incompatible h1:7qlOGliEKZXTDg6OTjfoBKDXWrumCAMpl/TFQ4/5kLM=
-github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
 github.com/dvsekhvalnov/jose2go v0.0.0-20200901110807-248326c1351b h1:HBah4D48ypg3J7Np4N+HY/ZR76fx3HEUGxDU6Uk39oQ=
 github.com/dvsekhvalnov/jose2go v0.0.0-20200901110807-248326c1351b/go.mod h1:7BvyPhdbLxMXIYTFPLsyJRFMsKmOZnQmzh6Gb+uquuM=
 github.com/form3tech-oss/jwt-go v3.2.3+incompatible h1:7ZaBxOI7TMoYBfyA3cQHErNNyAWIKUMIwqxEtgHOs5c=