Don't show user_update_permission in user form

Remove it from the supported_permission_strings list so that it doesn't appear
in the GUI. This was introduced in: 505a51a93cbb4d60ccc857a8e21b5b00ccb43242

We moved the check for unsupported permissions to the rake task so that
SSOPushCredential could still use grant_permissions to add the (supposedly)
unsupported permission.

This was easier than trying to make the supported permissions list vary based
on whether or not the user is the SSOPushUser.
Merge pull request #96 from alphagov/hide_user_update_permission

Don't show user_update_permission in user form