Remove extra LOCAL_CXX_STL := none

This was always being set in cc.Module.AndroidMk(), so we don't need to
declare it again. This was causing problems for cc_binary modules that
were using the NDK, since LOCAL_NDK_STL_VARIANT must be used instead.

Test: Manually put ifneq ($(LOCAL_CXX_STL),none); $(error) checks
Change-Id: If937b7b5ad30936e08c99f62ccf83b78d196dbfe