refactor(common): delete the Double Ratchet implementation nothing uses

311 lines of Signal Double Ratchet and 167 lines of tests for it, with no
caller: group chat is a key per group, per epoch, per device, and a ratchet was
ruled out for it on the record — a node that serves history to devices which
were not present has to hand out each chain's earliest key, which is forward
secrecy of zero.

Deleted for the same reason `senderkeys.py` was: an implementation kept for a
use nobody has reads as an alternative somebody may reach for, and its cost is
paid at every refactor that has to keep it compiling. The four comments that
mention a ratchet keep doing so — they explain why this is not one, which is
the part worth keeping.

Common suite 158 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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