[RISCV] Merge rv64zbkb-valid.s and rv64zbkb-only-valid.s. NFC

Based on the existing naming "only" tests are used for rv32 instructions
that don't exist in rv64. rv32 tests without "only" are for instructions
that are in both rv32 and rv64. The rv64 tests are for instructions
that are only in rv64.

Both of these test files have instruction encodings that are only
valid in rv64 so they can be the same file.