Remove DOMContentLoaded Eventlistener

What does this MR do?

This Merge Request solves these issues: 1 2 3 4 5 6

The DOMContentLoaded has been removed, it's no longer needed and slowing down the rendering of the page. See this epic for reference.

Screenshots and URL's

Issue: 1.
GDK Link: http://gitlab.localdev:3000/twitter/typeahead-js/-/requirements_management/requirements

Issue: 2 Could not be reproduced by the changes are minor and can be assumed save.

Issue: 3.
GDK Link: http://gitlab.localdev:3000/admin/push_rule

Issue: 4.
GDK Link: http://gitlab.localdev:3000/admin/users/$YOURUSERNAME/edit

Issue: 5.
GDK Link: http://gitlab.localdev:3000/admin/groups/new

Issue: 6.
GDK Link: http://gitlab.localdev:3000/admin/groups/$YOURGROUPNAME/edit

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