[4.2] Fix fatal error on webauthn plugin

[joomdonation]

Pull Request for Issue # .

Summary of Changes

PR #38060 removed $app property from the class, so using $this->app in the layout causes fatal error. We need to use $this->getApplication() to get application object instead.

I also removed an un-used use statement in the plugin layout code.

Testing Instructions

I don't have experience with using the plugin, so please ignore if my testing instructions are wrong

1. Use Joomla 4.2 nightly build
2. Make sure Multi-factor Authentication - Web Authentication plugin is enabled
3. Login to administrator area of your site, edit an user account, look at Multi-factor Authentication tab, click on Add a new Web Authentication button.

Actual result BEFORE applying this Pull Request

Get fatal error: Call to a member function getDocument() on null

Expected result AFTER applying this Pull Request

Error is gone, the page loads OK

[heelc29]

I have tested this item ✅ successfully on a5042e1

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38132.}

[richard67]

I have tested this item ✅ successfully on a5042e1

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38132.}

[richard67]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38132.}

[richard67]

RTC !!!

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38132.}

[laoneo]

Thanks!