repos: Deprecate enabled/disabled state

Created by: keegancsmith

We can only do this once all code hosts are migrated to the new syncer (#2025 (closed)). This issue has been optimistically assigned to milestone 3.4.

• Deprecate GraphQL mutations and fields

For 3.6

• Remove field from internal types
• Drop enabled column https://github.com/sourcegraph/sourcegraph/issues/3971

@sourcegraph/core-services