MSCC: delete obsolete reference to MSCC_BITBANG_SPI_GPIO

Remove "select MSCC_BITBANG_SPI_GPIO" since Kbuild option was deleted
back in commit ace9c103:

  commit ace9c103
  Author: Lars Povlsen <lars.povlsen@microchip.com>
  Date:   Tue Jan 8 10:38:35 2019 +0100

    mips: gpio: mscc: Obsoleted gpio-mscc-bitbang-spi.c

arch/mips/mach-mscc/Kconfig

@@ -29,7 +29,6 @@ config SOC_OCELOT
 config SOC_LUTON
 	bool "Luton SOC Family"
 	select SOC_VCOREIII
-	select MSCC_BITBANG_SPI_GPIO
 	help
 	  This supports MSCC Luton family of SOCs.