cmake: Remove FindRSSDK

Follow-up to PR #7371 (commit a1a98f9cfd).

We forgot to remove the Finder that was only used for that plugin. Let's
remove it now.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f