Revert "MSC3061: Sharing room keys for past messages (#3061)"
This reverts commit 6a3ab1d64cdf9f429a86b0e9d4df927dd36f0280.
Per https://matrix.org/blog/2024/10/security-disclosure-matrix-js-sdk-and-matrix-react-sdk/
and https://github.com/matrix-org/matrix-spec-proposals/pull/3061#issuecomment-2444845098,
we are removing the merged status of this PR.