Special:UserLogin: Don't show login button when not required

Special:UserLogin: Don't show login button when not required

If no AuthenticationRequest requires a separate login button, it
shouldn'tbe visible. This is, for example, the case, when only
link providers are used, that require the user to redirect to a third
party site, as it usually just shows a single submit button.

In this case, the login button is still visible because of other additional
fields, such as the remember  me button. This change checks each primary
authentication provider, if it provides its provide his own submit
button or not, and if so, removes the login button completely.

Bug: T141471
Change-Id: Ib18a69582cb3f79d438ab009d8755f0d5e415bcb