usb: usbtmc: Remove rigol_quirk

All T&M instruments should also work with rigol_quirk = 1 code path.
So remove unnecessary code in rigol_quirk = 0 code path to simplify the driver.

Tested-by: Dave Penkler <dpenkler@gmail.com>
Reviewed-by: Steve Bayless <steve_bayless@keysight.com>
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>