[doc] Remove the ngerman in refinement.tex.

Unfortunately, the referred English document is no longer available.
This should fix FS#1368 fo future releases.
(cherry picked from commit 9d6a1fbd2c3a0fefc37b680ded2dac4c73b5550a)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>