Epic create service cleanup

What does this MR do?

CreateService created new epics with whitelisted params, but at the same time this service inherits from IssuableBaseService which updates Issuable with all params passed when initializing the service.

To avoid impression that only whitelisted params are assigned, we process date params directly and let IssuableBaseService assign these params.

Because parms filtering is done on upper layer and UpdateService doesn't do additional params whitelisting either and existing whitelisting doesn't have any effect, I chose to remove this whitelisting.

Screenshots

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