spelling.txt: Remove some duplicate entries

Found when Colin King fixed a typo for falied/failed
and a git grep showed 2 entries in this file.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>