Remove unnecessary username option
Remove unnecessary username option

The -u argument to cf login is unnecessary, as the username is determined by
the email returned from OpenID Connect

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf