Issue #2256877 by znerol: Remove the API function to check if a cache bin is empty.

core/lib/Drupal/Core/Cache/BackendChain.php

core/lib/Drupal/Core/Cache/CacheBackendInterface.php

core/lib/Drupal/Core/Cache/DatabaseBackend.php

core/lib/Drupal/Core/Cache/MemoryBackend.php

core/lib/Drupal/Core/Cache/NullBackend.php

core/lib/Drupal/Core/Cache/PhpBackend.php

...odules/filter/lib/Drupal/filter/Tests/FilterAdminTest.php

...al/system/Tests/Cache/GenericCacheBackendUnitTestBase.php

core/modules/toolbar/toolbar.module

...l/Tests/Core/Cache/BackendChainImplementationUnitTest.php

core/tests/Drupal/Tests/Core/Cache/NullBackendTest.php