Remove after_initialize, before_validation for discussion_id on Note

What does this MR do?

Remove the after_initialize and before_validation callbacks for Note. after_initialize can cause performance degradation and shouldn't be used unless explicitly necessary.

Since we'd already overridden the discussion_id method, we shouldn't need to check discussion_id on every initialization.

Screenshots

Local benchmarks:

benchmark.txt

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team