Revert com code segregation

What does this MR do?

Based on discussion in &1969 (closed), this MR reverts the changes done for .com code segregation.

Reverted MRs:

1. Skeleton for segregation of GL com module: !17711 (merged)

2. Update the gitlab_host light setting to match the defaults: !18091 (merged)

3. Update the QA image Dockerfile to include 'config/light_settings.rb': !18061 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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