Remove Controller->getLoadedHandles()

Summary: Ref T7689. Removes this part of the `Controller->loadHandles()` + `Controller->getLoadedHandles()` mechanism.

Test Plan:
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  - Viewed Maniphest tasks with attached revisions and commits.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7689

Differential Revision: https://secure.phabricator.com/D12204

src/applications/phid/handle/pool/PhabricatorHandleList.php

@@ -57,6 +57,20 @@ final class PhabricatorHandleList
   }
 
 
+  /**
+   * Get a handle from this list if it exists.
+   *
+   * This has similar semantics to @{function:idx}.
+   */
+  public function getHandleIfExists($phid, $default = null) {
+    if ($this->handles === null) {
+      $this->loadHandles();
+    }
+
+    return idx($this->handles, $phid, $default);
+  }
+
+
 /* -(  Iterator  )----------------------------------------------------------- */