refactor(common): delete the sender-key implementation nothing uses

`senderkeys.py` and its 13 tests implemented Signal-style sender keys, and
production has never called them: chat is a key per group, per epoch, per
device, derived by name. The reasoning that ruled the ratchet out stays where it
belongs — in `chatbox.py`, at the top of the module that replaced it — because
the argument is the useful part, and it now stands on its own instead of
pointing at a file to compare against.

Kept code that nothing calls is worse than absent code: it reads as an
alternative somebody may reach for, and it has to be maintained past every
refactor to stay compiling, which is maintenance spent on a decision already
made.

The three comments naming `GroupSenderKeyStore` are rewritten to say the thing
they were illustrating.

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

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