Remove useless DEP_SCAN_DISABLE_REMOTE_CHECKS var

What does this MR do?

Remove the useless DEP_SCAN_DISABLE_REMOTE_CHECKS var. This setting is no longer useful since the redesign of Gemnasium analyzer to embed the advisory database #14630 (closed)

Most of the code and documentation updates and testing have been already done in %12.5

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.
  • Testing was done with #14630 (closed)
• 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

Refs #9953 (closed)