Blackfin: remove "config IP_CHECKSUM_L1"

The only user of Kconfig symbol IP_CHECKSUM_L1 got removed in v2.6.33,
with commit ddf9ddac ("Blackfin: convert
to generic checksum code"). We can remove the Kconfig entry for this
unused symbol now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>