Libvpx dropped armv6 support sometime after the aom fork.

We don't intend to support this platform, which is likely
too slow in any case. Remove the assembly and intrinsics
optimized routines, their tests, cpu feature detection,
and rtcd specialization for this instruction set extension.

Change-Id: If44ec28e5ddafc6af179c5d1982ac7e81fe54d5e