SwDoc::GetTxtCollFromPool: fix Footnote paragraph style

Commit 5cf62a6ab450c25eb07a1ec08c857f17f674ef67 (First modification on
the default template for the Writer, 2013-11-04) probably wanted to
ensure that the footnote field (which is usually just a number) is
superscript, but instead it modified the footnote itself. Having the
footnote text itself as superscript doesn't make sense by default.

Change-Id: Ic990d0afca356c2723eee0764b86b9c8c57c14b7