Remove old cloud licenses upon sync or activation

What does this MR do?

Related to #333870

This MR fixes 3 bugs related to syncing or activating a cloud licenses:

• Fixes a bug in the SyncSeatLinkRequestWorker where the current license was not being deleted when the new license was created. This was happening despite being tested in specs but did not work in development. This appears to be a problem with the way License.current uses caching. The .destroy! method did not actually delete the record from the DB outside of the test environment. In this MR, the fix instead deletes all existing cloud licenses uses delete_all.
• Applies a similar fix for removing old cloud licenses to the ActivateService. Previously if an instance attempted to activate multiple time, this would lead to multiple cloud licenses.
• Fixes a bug where new licenses created in the ActivateService were not created with the cloud attribute.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • I did not include a changelog because these changes are behind a feature flag.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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