Just remove redundant wxDocument::SetDocumentTemplate() call.

This is already done from InitDocument() called from
wxDocTemplate::CreateDocument().

Closes #15467.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74759 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775