staging: r8188eu: remove unused PHY_GetTxPowerLevel8188E()

Function PHY_GetTxPowerLevel8188E() is unused and it is the only user
of function phy_TxPwrIdxToDbm(), remove both.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210908223903.10516-2-straube.linux@gmail.com


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