net: ethernet: mediatek: ppe: Remove the unused function mtk_foe_entry_usable()

The function mtk_foe_entry_usable() is defined in the mtk_ppe.c file, but
not called elsewhere, so delete this unused function.

drivers/net/ethernet/mediatek/mtk_ppe.c:400:20: warning: unused function 'mtk_foe_entry_usable'.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2409
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>