math/coq: Remove USES=emacs and associated flavors

Upstream removed all elisp files in 2018:
https://github.com/rocq-prover/rocq/commit/41d597866d4f79fe5109c25c6f5cc57d0ebf7f0f

Since there will no longer be coq-emacs_* packages, add an UPDATING
entry with migration instructions.

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