remove unused variable
previous-sibling falls back to previous section

This makes `magit-goto-previous-sibling-section' symmetric with
`magit-goto-next-sibling-section'.

It was falling back to parent instead, which meant that
`magit-goto-previous-sibling-section' got "stuck" in the lines between
sections (they belong to top status section so there is no parent or
siblings).
Merge pull request #1001 from npostavs/goto-sibling

`magit-goto-previous-sibling-section` gets "stuck" between sections