`magit-stash-mode{,-map}': remove

Commit 8618430b added `magit-stash-mode-map', but neither it nor
`magit-stash-mode' (which the former is supposed to be used by)
serve any purpose:  `magit-show-stash' uses `magit-diff-mode'.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #938 from praet/stash-mode

`magit-stash-mode{,-map}': remove