DO NOT MERGE - Remove deprecated image defines

libvpx has always supported the VPX_ prefixed versions of these defines.
The unprefixed versions have been removed in the most recent release.

https://chromium.googlesource.com/webm/libvpx/+/9cdaa3d72eade9ad162ef8f78a93bd8f85c6de10

BUG=23452792

Change-Id: I8a656f2262f117d7a95271f45100b8c6fd0a470f
