Groups no longer duplicated if set is created with create_group_count

create_group_count was not being reset after a successful sync

Fixes CNVS-15204

Test Plan:

- I- n a course, open the new group set dialog
- Check allow self sign-up, enter a number for groups to create now
- Save the group set.
- Click the gear to edit the group set.
- Save the group set.
- Refresh the page.
- Note the duplicated groups in the set.

Change-Id: I493c1980dab4d15ce3a6a39b0c469e6c9bd39a28
Reviewed-on: https://gerrit.instructure.com/42582
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>

app/coffeescripts/views/groups/manage/GroupCategoriesView.coffee

@@ -123,6 +123,7 @@ define [
         @reorder()
         @refreshTabs()
         @$tabs.tabs active: @tabOffsetOfModel(cat)
+        cat.set "create_group_count", null
       @createView.model = cat
       @createView.open()