Force empty titles for WebPreferences pages for XAR extensions

Following votes at https://forum.xwiki.org/t/drop-the-maven-xar-plugin-requirement-for-setting-the-title-for-webpreferences-pages/17515,

The XAR maven plugin currently enforce the following title for WebPreferences pages: $services.localization.render('admin.preferences.title'), which is inconsistent with what the platform does (A sheet associated to WebPreference pages sets the title to $services.localization.render('administration.title.page')).

Because

• we shouldn’t require users to hardcode some arbitrary translation string they should not have to care about in their projects
• this is not needed (anymore?) anyway
• it’s causing inconsistencies between WebPreferences pages created on the instance directly, and those imported from a XAR package

We should replace this requirement with requiring empty titles to let the platform handle WebPreferences titles automatically.