[#3338] Removing delete_empty details because this is first being committed to 2.3

Since the original PR had a mix of changes that should apply to 2.3+ and 2.5+, I've
merged the entire PR into 2.3 and this commit removes the 2.5+ specific documentation.
After merging into master (i.e. 2.5), I'll reverse this commit to put the delete_empty details back.

Author: weaverryan

reference/forms/types/collection.rst

@@ -19,7 +19,6 @@ forms, which is useful when creating forms that expose one-to-many relationships
 |             | - `allow_delete`_                                                           |
 |             | - `prototype`_                                                              |
 |             | - `prototype_name`_                                                         |
-|             | - `delete_empty`_                                                           |
 +-------------+-----------------------------------------------------------------------------+
 | Inherited   | - `label`_                                                                  |
 |             | - `label_attr`_                                                             |
@@ -336,16 +335,6 @@ If you have several collections in your form, or worse, nested collections
 you may want to change the placeholder so that unrelated placeholders are not
 replaced with the same value.
 
-delete_empty
-~~~~~~~~~~~~
-
-**type**: ``Boolean`` **default**: ``false``
-
-If you want to explicitly remove entirely empty collection entries from your
-form you have to set this option to true. However, existing collection entries
-will only be deleted if you have the allow_delete_ option enabled. Otherwise
-the empty values will be kept.
-
 Inherited options
 -----------------