MAKEALL: drop obsolete mx31pdk_nand target

The mx31pdk can boot only from NAND and the target was
already updated in boards.cfg. mx31pdk_nand is obsolete
and is dropped.

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/MAKEALL b/MAKEALL
index 95b7cd3..2a77857 100755
--- a/MAKEALL
+++ b/MAKEALL
@@ -316,7 +316,6 @@
 	imx31_phycore		\
 	imx31_phycore_eet	\
 	mx31pdk			\
-	mx31pdk_nand		\
 	smdk6400		\
 "