wifi: rtw88: remove unused rtw_pci_get_tx_desc function

clang with W=1 reports
drivers/net/wireless/realtek/rtw88/pci.c:92:21: error:
  unused function 'rtw_pci_get_tx_desc' [-Werror,-Wunused-function]
static inline void *rtw_pci_get_tx_desc(struct rtw_pci_tx_ring *tx_ring, u8 idx)
                    ^
This function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230320233448.1729899-1-trix@redhat.com