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

POST support for sample lwmon board was removed in commit e5d30786.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>B

post/Makefile

@@ -13,6 +13,6 @@ obj-$(CONFIG_PPC) += lib_powerpc/
 obj-$(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