authz: Remove sub-repo SupportedChecker

Created by: ryanslade

Sub-repo permissions are additional filters applied after we have already checked our existing repo permissions.

It is therefore not necessary to check whether sub-repo perms are enabled for a repo. If they are, we'll have data in the sub_repo_permissions table, which we plan to cache. If not, we assume that the user has access to the whole repo based on our traditional repo permissions checks.

SubRepoPermsClient was also exported as it is bad practice to have constructors return private structs.

Thanks @bobheadxi for spotting this simplification!