Remove MSC3215 (Report to Moderators) labs feature (element-hq#33492)

* Remove MSC3215 (Report to Moderators) labs feature

T&S does not believe this is used, and intends to replace it during Reporting v2 anyway.

Original MSC: matrix-org/matrix-spec-proposals#3215

New (WIP) MSC: matrix-org/matrix-spec-proposals#4468

(This PR/commit doesn't add MSC4468 support because it's unclear if we should use MSC4468 directly or via another endpoint like MSC4457)

[MSC4457]: matrix-org/matrix-spec-proposals#4457

* Remove unused state: isRoomEncrypted

Author: turt2live