igb: Cleanup unused declarations

e1000_init_function_pointers_82575() is never implemented and used since
commit 9d5c8243 ("igb: PCI-Express 82575 Gigabit Ethernet driver").
And commit 9835fd73 ("igb: Add new function to read part number from
EEPROM in string format") removed igb_read_part_num() implementation.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>