Remove sixtap/bilinear 4x4 neon implementations

These implementations rely on casting the pointers to load the data.
Clang implemented optimizations which automatically add alignment hints
to such loads. The 4x4 filters do not guarantee the necessary alignment
so the resulting assembly is broken.
https://llvm.org/bugs/show_bug.cgi?id=24421

BUG=webm:817
BUG=webm:892

Change-Id: I608885299f1f86ff83653b65e0e40d0ae87fb3fe
3 files changed