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

Change-Id: Idae747eb9f6e71a41a63c6b326af36dc52f5446d