Remove Snowplow controller concern

What does this MR do?

As part of the Snowplow standardization effort (&5218 (closed)), we are no longer supporting use of the controller concern. We instead encourage the use of Gitlab::Tracking.event directly. The controller concern was not documented, so a docs update is not needed.

Related to #321600 (closed)

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because this is not user-facing.
• [-] Documentation (if required)
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• 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] 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