And remove the AVOID_PROBING flag, given it's the last av1 decoder to be tested
either way.
This fixes a regression introduced in 1652f249,
where even if forcing the native av1 decoder, if another decoder was present,
like libdav1d or libaom-av1, they'd be used for probing and some fate tests
would have different results.
Signed-off-by: James Almer <jamrial@gmail.com>