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: Ib02073f42d545e6c08f9bd4a4fc868e3be886c1b