ldpc: drop fake interop/ccsds131 oracle
The Python generator built the AR4JA H matrix from CCSDS 131.0-B-4 and
performed GF(2) Gaussian elimination in numpy to produce reference
codewords. Per the interop-testing skill, that's an internal verification
of the OCaml encoder against another implementation of the same spec, not
an interop oracle. Library roundtrip + decoder coverage stays in
test_ldpc.ml.