Web IDE - remove unused actions (mappings)

What does this MR do?

Removing unused action closeAllFiles and unused mapped actions stageChange and stageAllChanges in two separate commits.

I found this dead code when walking through the Web IDE codebase, this change is not related to any feature currently being implemented.

This MR is my first GitLab frontend MR and I'd like to see the MR process in action on a small scale.

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

I tested locally that closing files, discarding changes and committing changes works (only as a sanity check). I wasn't able to find any usages in the codebase and so I'm not expecting any additional risks.

• [-] 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

Does not apply