Core patch no longer necessary in Drupal 7.22

The core patch was committed to Drupal 7 at #1821906-38: Allow more Field API public API functions to act on a single field within an entity.

As soon as Drupal 7.22 ships, we'll be able to remove the core patch.

(Also see #1931766: README is wrong: core patch must still be applied when using Drupal 7.20.)