app-i18n/man-pages-de: Bump to version 2.10. Removed version 2.9

app-i18n/man-pages-de: Bump to version 2.10. Removed version 2.9

- Bumped to EAPI-7
- removed man1/su.1.po file as it collides with sys-apps/shadow package
- put all to be removed files into a local array
- Remove "gzip --best" call from Makefile.am instead of Makefile.in
  because eautoreconf call comes at a later point effectively
  reverting the change again
- Improved readability of case calls
- Moved src_unpack() stuff into src_prepare()
- Fix man-page links when xz compression is being used

Bug: https://bugs.gentoo.org/670092
Package-Manager: Portage-2.3.60, Repoman-2.3.12
Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>