[source-linkedin-ads] Removed sponsored content stream

### Connector Name

LinkedIn Ads

### Connector Version

0.6.4

### What step the error happened?

Other

### Relevant information

Ever since the following pull request was merged (https://github.com/airbytehq/airbyte/pull/26372), the `sponsored_content` stream, provided by the `AdDirectSponsoredContents` class , is no longer supported in this plugin. The associated classes were simply removed without any comment on the corresponding PR of why this was done. This leads me to believe that the stream was removed accidentally and never put back. You can verify in the referenced PR in this issue that all streams were migrated except the `sponsored_content` stream who's endpoint still exists today (as can be checked [here](https://learn.microsoft.com/en-us/linkedin/marketing/integrations/ads/advertising-targeting/create-and-manage-video?view=li-lms-2024-02&tabs=http#finders)) and returns relevant data.

### Relevant log output

Code removed so no log output.

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