staging: rtl8188eu: remove HW_VAR_TXPAUSE

The "HAL layer variable" HW_VAR_TXPAUSE is not used in this driver.
Remove its define and the code for setting and for reading a value.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210725155902.32433-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>