post: Remove references to scrapped "netta" board.

Given that README.scrapyard shows scrapping of netta boards:

netta2           powerpc     mpc8xx         c51c1c9a    2014-07-07
netta            powerpc     mpc8xx         c51c1c9a    2014-07-07

delete netta example from POST tests.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
diff --git a/post/Makefile b/post/Makefile
index 92024e1..78ede8c 100644
--- a/post/Makefile
+++ b/post/Makefile
@@ -13,6 +13,6 @@
 obj-$(CONFIG_MPC83xx) += cpu/mpc83xx/
 obj-$(CONFIG_8xx) += cpu/mpc8xx/
 obj-$(CONFIG_4xx) += cpu/ppc4xx/
-ifneq ($(filter lwmon5 netta pdm360ng,$(BOARD)),)
+ifneq ($(filter lwmon5 pdm360ng,$(BOARD)),)
 obj-y += board/$(BOARD)/
 endif