MSCC: delete obsolete reference to MSCC_BITBANG_SPI_GPIO

Remove "select MSCC_BITBANG_SPI_GPIO" since Kbuild option was deleted
back in commit ace9c103df2875d2b435dbd7b36618020edfd1c0:

  commit ace9c103df2875d2b435dbd7b36618020edfd1c0
  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
diff --git a/arch/mips/mach-mscc/Kconfig b/arch/mips/mach-mscc/Kconfig
index 34584a1..affc472 100644
--- a/arch/mips/mach-mscc/Kconfig
+++ b/arch/mips/mach-mscc/Kconfig
@@ -29,7 +29,6 @@
 config SOC_LUTON
 	bool "Luton SOC Family"
 	select SOC_VCOREIII
-	select MSCC_BITBANG_SPI_GPIO
 	help
 	  This supports MSCC Luton family of SOCs.