Miner v12: cleanup: drop all support for v1 window PoSt proof types

[arajasek]

There is partial support for v1 PoSt proof types in v11 that must be dropped in v12. Specifically, we must:

• Remove v1 proof types from the policy's valid_post_proof_type
• Remove special-case checks in the miner constructor
• Remove the special v19 check in submit_windowed_post that allows for v1 proof types to be submitted in v19

[jennijuju]

This is a nice to have for nv21/ actor v12 (and in general we should avoid keep postponing clean ups).

[jennijuju]

@elmattic thank you for taking this one!