bonding: remove unused bond_is_first_slave and bond_is_last_slave macros

bonding: remove unused bond_is_first_slave and bond_is_last_slave macros

Since commit 2884bf72fb8f ("net: bonding: fix use-after-free in
bond_xmit_broadcast()"), bond_is_last_slave() was only used in
bond_xmit_broadcast().  After the recent fix replaced that usage with
a simple index comparison, bond_is_last_slave() has no remaining
callers.  bond_is_first_slave() likewise has no callers.

Remove both unused macros.

Signed-off-by: Xiang Mei <xmei5@asu.edu>
Link: https://patch.msgid.link/20260404220412.444753-1-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>