graphics/libavif: Remove LIBYUV option

libavif requires libyuv, either libyuv port or the bundled one (a simplified version).
However, graphics/libyuv is too old (1789) while the bundled one is 1880.
Therefore, we remove the LIBYUV option and use the bundled one for now.

from build log:

• Checking for module 'libyuv'
• Found libyuv, version 0.0.1789
• libavif: libyuv (1789) found; libyuv-based fast paths enabled.
• libavif: some libyuv optimizations require at least version 1813 to work.