rtwn: disable a workaround introduced earlier for RTL8192CU TX performance

I'm unable to reproduce the original problem with my RTL8192CU USB
devices with the current codebase and I can't find any reference
to what this power register is doing - I see it defined in drivers,
but it's not described or used anywhere.

This reverts 7f740971658d71c1ee95ee68032b4696c1684845 -
rtwn_usb(4): fix Tx instability with RTL8192CU chipsets

In any case being able to do higher rate RTS/CTS is beneficial.

Local testing:

• rtl8192cu, STA mode, TX/RX testing

PR: 233949

Differential Revision: https://reviews.freebsd.org/D48026
Reviewed by: imp