Drop unused EXTERNAL_MRC_BLOB

The Kconfig variable EXTERNAL_MRC_BLOB is not used.
Drop it.

Change-Id: I3caa5c2b6bcf5d2c13b6987da8ab3987bad0e506
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/3829
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/mainboard/intel/baskingridge/Kconfig

@@ -12,7 +12,6 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select HAVE_ACPI_RESUME
 	select HAVE_SMI_HANDLER
 	select CHROMEOS
-	select EXTERNAL_MRC_BLOB
 
 config MAINBOARD_DIR
 	string