TaxonomyHierarchySelection checks for target_bundles in incorrect location

Problem/Motivation

TaxonomyHierarchySelection does this:

      $bundles = $this->entityTypeBundleInfo->getBundleInfo('taxonomy_term');
      $bundle_names = array_keys($bundles);

      // If we have specific handler settings, use them.
      if (isset($this->configuration['handler_settings'])) {
        $handler_settings = $this->configuration['handler_settings'];
        $bundle_names = $handler_settings['target_bundles'];
      }

handler_settings never exists in this context

Steps to reproduce

Create some vocabularies with thousands of taxonomy terms
Create a small editorial section vocab
Notice all terms in every vocab are loaded when using entity_autocomplete widget for the WBA field
Get an OOM if you have a low memory ceiling

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes