R1:c40be811ea9b

Fix isReadable() and isWritable() in SearchService

Summary:
Ref T12450. Minor cleanup:

- setRoles() has no callers.
- getRoles() has no callers (these two methods are leftovers from an earlier iteration of the change).
- The `hasRole()` logic doesn't work since nothing calls `setRole()`.
- `hasRole()` has only `isreadable/iswritable` as callers.
- The `isReadable()/isWritable()` logic doesn't work since `hasRole()` doesn't work.

Instead, just check if there are any readable/writable hosts.