Remove v16 docs as they are not currently needed

Author: rickbutterfield

16/umbraco-forms/SUMMARY.md

@@ -50,7 +50,6 @@
   * [Adding A Field Type To Umbraco Forms](developer/extending/adding-a-fieldtype.md)
     * [Excluding a built-in field](developer/extending/excluding-a-built-in-field.md)
   * [Adding A Prevalue Source Type To Umbraco Forms](developer/extending/adding-a-prevaluesourcetype.md)
-    * [Adding a UI Builder repository as a prevalue source](developer/extending/uibuilder-repository-prevalue.md)
   * [Adding A Workflow Type To Umbraco Forms](developer/extending/adding-a-workflowtype.md)
   * [Adding An Export Type To Umbraco Forms](developer/extending/adding-a-exporttype.md)
   * [Adding a Magic String Format Function](developer/extending/adding-a-magic-string-format-function.md)