server: return empty org_name for resultField

[YangKeao]

What problem does this PR solve?

TiDB should return an empty org_name for an expression.

Issue Number: close #45145

What is changed and how it works?

1. Remove some useless fields in the ast.ResultField. They are only assigned value but never read (but I'm not sure whether the codes in other repo will read these things)
2. Add a new field to indicate whether this column should return an empty org_name. I didn't set an "" to the column info directly to avoid some unknown bugs (but I'm not sure whether they exist).

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

Previously, TiDB returns the alias name as the `org_name` in mysql protocol. From now on, TiDB will return empty string as the `org_name` if this field is an expression.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CbcWestwolf, hawkingrei

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [CbcWestwolf,hawkingrei]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-04 09:11:31.277256272 +0000 UTC m=+108723.210889691: ☑️ agreed by CbcWestwolf.
• 2023-07-05 03:04:14.205848546 +0000 UTC m=+173086.139481980: ☑️ agreed by hawkingrei.