Remove two surplus strings from the language pack

lang/en/theme_boost_union.php

@@ -421,11 +421,9 @@
 $string['backtotopbuttonsetting'] = 'Back to top button';
 $string['backtotopbuttonsetting_desc'] = 'With this setting a back to top button will appear in the bottom right corner of the page as soon as the user scrolls down the page. A button like this existed already on Boost in Moodle Core until Moodle 3.11, but was removed in 4.0. With Boost Union, you can bring it back.';
 // ... ... Setting: Scroll-spy
-$string['scrollspy'] = 'Scroll-spy';
 $string['scrollspysetting'] = 'Scroll-spy';
 $string['scrollspysetting_desc'] = 'With this setting, upon toggling edit mode on and off, the scroll position at where the user was when performing the toggle is preserved.';
 // ... ... Setting: Activity navigation
-$string['activitynavigation'] = 'Activity navigation';
 $string['activitynavigationsetting'] = 'Activity navigation elements';
 $string['activitynavigationsetting_desc'] = 'With this setting the elements to jump to the previous and next activity/resource as well as the pull down menu to jump to a distinct activity/resource become displayed. UI elements like this existed already on Boost in Moodle Core until Moodle 3.11, but were removed in 4.0. With Boost Union, you can bring them back.';