post: Remove reference to deleted "lwmon" board from Makefile

POST support for sample lwmon board was removed in commit e5d3078622.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>B
diff --git a/post/Makefile b/post/Makefile
index 2fa6f8a..92024e1 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 lwmon lwmon5 netta pdm360ng,$(BOARD)),)
+ifneq ($(filter lwmon5 netta pdm360ng,$(BOARD)),)
 obj-y += board/$(BOARD)/
 endif