wifi: rtl8xxxu: Remove copy-paste leftover in gen2_update_rate_mask

It looks like a leftover from copying rtl8xxxu_update_rate_mask,
which is used with the gen1 chips.

It wasn't causing any problems for my RTL8188FU test device, but it's
clearly a mistake, so remove it.

Fixes: f653e690 ("rtl8xxxu: Implement basic 8723b specific update_rate_mask() function")
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/d5544fe8-9798-28f1-54bd-6839a1974b10@gmail.com