Android.mk: remove some build-log spam

This line always prints when building master branch, it's not
particularly useful.

system/sepolicy/Android.mk:77: warning: BOARD_SEPOLICY_VERS not
specified, assuming current platform version

Test: build
Change-Id: I52f8dc2a77966bc0c21168b1339f3029185e5339
diff --git a/Android.mk b/Android.mk
index e3ae00c..f5f277d 100644
--- a/Android.mk
+++ b/Android.mk
@@ -74,7 +74,6 @@
 # policy is to be based. If unspecified, this will build against the current
 # public platform policy in tree
 ifndef BOARD_SEPOLICY_VERS
-$(warning BOARD_SEPOLICY_VERS not specified, assuming current platform version)
 # The default platform policy version.
 BOARD_SEPOLICY_VERS := $(PLATFORM_SEPOLICY_VERSION)
 endif