CMake experiment flag clean up.
CMake experiment flag clean up.

Alpha sort the experiments group in
aom_config_defaults.cmake to avoid future duplication
issues, and:

- Remove ALTREF2.
- Remove the duped CDEF line.

BUG=aomedia:813

Change-Id: Ib14172c89dd3934dbf475bbfc581d017acdba2d8

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom