Remove code related to "context from configuration" that was deprecated in Drupal 9

#2273381: Convert ContextAwarePluginBase to traits calls for the deprecation of passing contexts via configuration. This issue is a placeholder to remove the code relating to this deprecation.

The following items require attention:

1. Remove the entire \Drupal\Core\Plugin\ContextAwarePluginBase class and file.
2. Remove this snippet in \Drupal\Core\Plugin\ContextAwarePluginTrait::getContext():
   

       if (!$this->context) {
         if ($this instanceof ConfigurableInterface) {
           $configuration = $this->getConfiguration();
         }
         else {
           $reflection = new \ReflectionProperty($this, 'configuration');
           $reflection->setAccessible(TRUE);
           $configuration = $reflection->getValue($this);
         }
   
         if (isset($configuration['context'])) {
           @trigger_error('Passing context values to plugins via configuration is deprecated in drupal:9.1.0 and will be removed before drupal:10.0.0. Instead, call ::setContextValue() on the plugin itself. See https://www.drupal.org/node/3120980', E_USER_DEPRECATED);
           foreach ($configuration['context'] as $key => $value) {
             $context_definition = $this->getContextDefinition($key);
             $this->context[$key] = new Context($context_definition, $value);
           }
         }
       }