Removing old release post

Why is this change being made?

Two features for Defend:Threat Insights had very similar content so were combined into a single release post entry. This MR removes the no longer needed unreleased release post item .yml file so it doesn't keep popping up in master.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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