Clean up power actor MinerAboveMinPowerCount state for miners with invalid sector sizes

[anorth]

In #1345 we unintentionally lost a check that prevented creation of new miners with proofs types that are not supported by the network. A few miners with unsupported sector sizes were created on mainnet. These miners can never commit a sector so are essentially DOA. However, the 2KiB miners incremented st.MinerAboveMinPowerCount because the consensus minimum power for that sector size is zero.

This is not a practical problem for mainnet, but it would still be good to clean up the state to properly reflect that these miners are ineligible to produce a block. It's a logical flaw to count zero-powered miners towards the ConsensusMinerMinMiners threshold. I might suggest removing their claims from the power table might be a good way to do it (and updating the MinerAboveMinPowerCount to be consistent). But other approaches might work too.

See also #1421

[ZenGround0]

@laudiacay this is closely related to #1474 so is a nice one to work on after that.