[IPV4]: Remove ip_fib_local_table and ip_fib_main_table defines.

There are only 2 users and it doesn't hurt to call fib_get_table
instead, and it makes it easier to make the fib network namespace
aware.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>