orangefs: remove unused 'diff' function

orangefs contains a helper function to calculate the difference
between two timeval structures. We are trying to remove all
instances of timespec from the kernel, and this one is not
used at all, so let's remove it now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>