Remove $wgEnableUserLoginExt from the code.

The variable has been removed as it was always `true`. The code
depending on the value has been removed:

* code executed on `true` stays
* code executed on `false` goes away
Merge pull request #5710 from Wikia/release-255-main-3348-remove-wgEnableUserLoginExt-completely

Remove $wgEnableUserLoginExt from the code.