Remove `autoExpandCollapsedDiffs` feature flag

What does this MR do?

For #243604 (closed) (fixes "Expand All" button not working)

Right now, the "Expand All" button that appears in the diffs header bar when any file is collapsed doesn't actually perform its action. It changes the values in state (which causes the button to disappear), but the files don't actually open.

This is due to the watcher on the file.viewer.collapsed property being modified to have a few clauses around the actual modification of the isCollapsed data property. The file will only expand if the user is not in file-by-file mode and the autoExpandCollapsedDiffs feature is not enabled.

This MR removes the autoExpandCollapsedDiffs feature flag so the only exclusion is if the user is viewing the diffs in file-by-file mode.

Screenshots

Before	After
simplescreenrecorder-2020-08-31_12.41.12

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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