ice: Remove two impossible branches on XDP Tx cleaning

The tagged commit started sending %XDP_TX frames from XSk Rx ring
directly without converting it to an &xdp_frame. However, when XSk is
enabled on a queue pair, it has its separate Tx cleaning functions, so
neither ice_clean_xdp_irq() nor ice_unmap_and_free_tx_buf() ever happens
there.
Remove impossible branches in order to reduce the diffstat of the
upcoming change.

Fixes: a24b4c6e

 ("ice: xsk: Do not convert to buff to frame for XDP_TX")
Suggested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20230210170618.1973430-4-alexandr.lobakin@intel.com