don't delete side comments when an entry is destroyed

test plan:
 * Create an entry in a discussion, and make some side comments on it
 * Delete the first entry
 * Verify that the side comments were not deleted as well.

Change-Id: I1eea75877e39f2df56fa4e0013c5bd1649407359
Reviewed-on: https://gerrit.instructure.com/9656
Reviewed-by: Brian Whitmer <brian@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

Author: zwily

app/models/discussion_entry.rb

@@ -158,7 +158,6 @@ def plaintext_message(length=250)
 
   alias_method :destroy!, :destroy
   def destroy
-    flattened_discussion_subentries.destroy_all
     self.workflow_state = 'deleted'
     self.deleted_at = Time.now
     save!

spec/models/discussion_entry_spec.rb

@@ -20,7 +20,7 @@
 
 describe DiscussionEntry do
 
-  it "should be marked as deleted when parent is deleted" do
+  it "should not be marked as deleted when parent is deleted" do
     topic = course.discussion_topics.create!
     entry = topic.discussion_entries.create!
 
@@ -31,8 +31,8 @@
     topic.discussion_entries.active.length.should == 2
     entry.destroy
     sub_entry.reload
-    sub_entry.should be_deleted
-    topic.discussion_entries.active.length.should == 0
+    sub_entry.should_not be_deleted
+    topic.discussion_entries.active.length.should == 1
   end
 
   it "should preserve parent_id if valid" do