[AX25]: Remove obsolete references to BKL from TODO file.

Given that there are no apparent calls to lock_kernel() or
unlock_kernel() under net/ax25, delete the TODO reference related to
that.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>