Clean up `QuotaRequest` and `QuotaInfo`.

Once quota limits are implementation complete, we should clean up the `QuotaInfo` and `QuotaRequest` proto along with other legacy code and tests. Specifically:

• Remove the experimental `limits` field in `QuotaInfo` and `QuotaRequest` (code in https://reviews.apache.org/r/65852/ and https://reviews.apache.org/r/65851/ and https://reviews.apache.org/r/65334/)
• Update the related `QuotaRequest` validation code and tests (code in https://reviews.apache.org/r/65784/ and https://reviews.apache.org/r/65785/)
• The `principal` field is no longer used. It was used to support the remove_quota acl which was already deprecated. (On second thought, while it is not used by the Mesos local authorizer, an external auth module could potentially depend on this. We need to keep this for backward compatibility.)