Remove prebuilt rules changelog reference

[shashank-elastic]

Summary

• Remove the reference of prebuilt rules changelog which is no longer maintained.
• Issue details https://github.com/elastic/ia-trade-team/issues/254

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[joepeeples]

We should also remove the source file for the page, since it will no longer be needed. Otherwise LGTM!

[shashank-elastic]

We should also remove the source file for the page, since it will no longer be needed. Otherwise LGTM!

Thanks @joepeeples also this branch somehow got the pattern of the protected branch and i am unable to merge latest into it! Will create a fresh PR all together closing this one.