Remove email address field from login

This removes the email prompt when you use docker login, and also removes the ability to register via the docker cli. Docker login, will strictly be used for logging into a registry server.

Signed-off-by: Ken Cochrane <kencochrane@gmail.com>

docs/reference/commandline/login.md

@@ -12,10 +12,9 @@ parent = "smn_cli"
 
     Usage: docker login [OPTIONS] [SERVER]
 
-    Register or log in to a Docker registry server, if no server is
+    Log in to a Docker registry server, if no server is
 	specified "https://index.docker.io/v1/" is the default.
 
-      -e, --email=""       Email
       --help               Print usage
       -p, --password=""    Password
       -u, --username=""    Username
@@ -27,10 +26,10 @@ adding the server name.
     $ docker login localhost:8080
 
 
-`docker login` requires user to use `sudo` or be `root`, except when: 
+`docker login` requires user to use `sudo` or be `root`, except when:
 
 1.  connecting to a remote daemon, such as a `docker-machine` provisioned `docker engine`.
-2.  user is added to the `docker` group.  This will impact the security of your system; the `docker` group is `root` equivalent.  See [Docker Daemon Attack Surface](https://docs.docker.com/security/security/#docker-daemon-attack-surface) for details. 
+2.  user is added to the `docker` group.  This will impact the security of your system; the `docker` group is `root` equivalent.  See [Docker Daemon Attack Surface](https://docs.docker.com/security/security/#docker-daemon-attack-surface) for details.
 
 You can log into any public or private repository for which you have
 credentials.  When you log in, the command stores encoded credentials in