Remove unused method shouldCopyWhenCreatingSimilarTask()

Summary:
Nothing in the codebase calls or reads this function.

If it was ever used in some implementation of a custom field, then it was likely in context of the `Similar Task` functionality removed in 52f7446eea.

Test Plan: Grep the codebase.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25677

src/applications/maniphest/field/ManiphestCustomField.php

@@ -15,17 +15,4 @@ abstract class ManiphestCustomField
     return new ManiphestCustomFieldNumericIndex();
   }
 
-  /**
-   * When the user creates a task, the UI prompts them to "Create another
-   * similar task". This copies some fields (e.g., Owner and CCs) but not other
-   * fields (e.g., description). If this custom field should also be copied,
-   * return true from this method.
-   *
-   * @return bool True to copy the default value from the template task when
-   *              creating a new similar task.
-   */
-  public function shouldCopyWhenCreatingSimilarTask() {
-    return false;
-  }
-
 }