Issue #2089461 by thedavidmeister, deneo, Alan D., longwave, dsdeiz, stpaultim: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/lib.

Issue #2081151 by Tim Bozeman, deneo, Pete B: Remove Unused local variables from the content_translation module.

Issue #2089461 by thedavidmeister, deneo, Alan D., dsdeiz, stpaultim: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/lib.

Issue #2081149 by areke, deneo, mrsinguyen: Remove Unused local variable  from /core/modules/content_translation/content_translation.module.

Issue #2080551 by beowulf1416, deneo: Remove Unused local variable  from /core/modules/search/lib/Drupal/search/SearchQuery.php.

Issue #2081131 by mrsinguyen, deneo2: Remove Unused local variable  from /core/modules/taxonomy/lib/Drupal/taxonomy/Tests/Views/TaxonomyTestBase.php.

Issue #2072593 by stevepurkiss, legolasbo, deneo: Remove Unused local variable  from /core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php.

Issue #2080113 by deneo, rhm50: Remove Unused local variable $plugin_id from /core/modules/system/lib/Drupal/system/Tests/Plugin/CacheDecoratorLanguageTest.php.