Remove cluster applications report and parser

What does this MR do?

This artifact does not have any user-visible impact anymore, since it only affected the now-removed applications tab on the cluster page. See &4280.

Instead, an API may be added in the future for interacting with cluster integrations. See #326565 (closed).

Split out of !63597 (closed) as it is a functional change that warrants its own MR (as opposed to dead code removal).

NOTE: The artifacts:reports:cluster_applications keyword will still be allowed in order to not break existing pipelines, but does nothing (besides uploading an unused report). The issue for its eventual removal is #333441 (closed)

Screenshots (strongly suggested)

Small screencast demonstrating that jobs producing the artifact do not fail:

2021-06-14_16-21-53

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.