cpu/intel/lga775: Do not select model_6ex CPU

Model 6ex are Core Solo and Core Duo CPUs (yonah) that never existed
with a LGA775 socket.

This reduces the size of the microcode from 180k to 168k.

Change-Id: Ic5b3d0e7c8009dab2dca477010c328274a818fed
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/17120
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/cpu/intel/socket_LGA775/Makefile.inc

@@ -1,4 +1,3 @@
-subdirs-y += ../model_6ex
 subdirs-y += ../model_6fx
 subdirs-y += ../model_f3x
 subdirs-y += ../model_f4x