Remove obsolete <machine/cpu-features.h>.

This is no longer used in the platform, and shouldn't be used in NDK. Apps
should use the NDK's cpu-features module, which supports (a) more specific
queries and (b) all Android architectures, not just 32-bit ARM.

Bug: http://b/18556103
Test: builds
Change-Id: I544ef267a6d7d887223186180c77d9ad0321e605