ice: Cleanup unused declarations

Since commit fff292b4 ("ice: add VF representors one by one")
ice_eswitch_configure() is not used anymore.
Commit 1b8f15b6 ("ice: refactor filter functions") removed
ice_vsi_cfg_mac_fltr() but leave declaration.
Commit a24b4c6e ("ice: xsk: Do not convert to buff to frame for
XDP_TX") leave ice_xmit_xdp_buff() declaration.

Commit 7cab44f1 ("ice: Introduce ETH56G PHY model for E825C
products") declared ice_phy_cfg_{rx,tx}_offset_eth56g(),
commit a1ffafb0 ("ice: Support configuring the device to Double
VLAN Mode") declared ice_pkg_buf_get_free_space(), and
commit 8a3a565f ("ice: add admin commands to access cgu
configuration") declared ice_is_pca9575_present(), but all these never
be implemented.

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>