Removes _checked from column name query

[yaoandrew]

This change is needed to support Edvard's config refactor. It will remove the "_checked" part of the query and return a collection of column names (transformers) to be used with the new configs here: vulcanize/mcd_transformers#154

[rmulhol]

@i-norden curious if any pitfalls jump out at you with these changes - namely, if they affect other things you're working on

[rmulhol]

Maybe we should update the expected column names in the test to omit _checked?

[i-norden]

@rmulhol the seed node stuff isn't concerned with the checked_headers columns, and the contract_watcher code and any of its derivative ContractTransformers (e.g. ENS domain_records and non-seed-node-based account transformers) don't use this repository interface so those will be unaffected. I do think this will necessitate changes to any other EventTransformers though, i.e. all the individual ENS event transformers here but that doesn't seem like an issue, they can continue to work with older versions of vulcanizedb and can be updated if need be. Thank you for checking!!