lib/Makefile: remove CONFIG_AVERAGE build rule

The Kconfig option AVERAGE and its implementation has been removed by
commit f4e774f5 ("average: remove out-of-line implementation").
Remove the dead build rule in lib/Makefile.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>