[RFC] Remove remaining crypto references

I've added a reference to #694 and #701 in `runtime/doc/editing.txt`, as I'm not knowledgeable enough in crypto to verify the claims being made in said discussions. That being said, if someone can provide a concise explanation for the crypto removal then I can add that instead.

It's also worth noting that I removed all crypto references in `runtime/doc/usr_23.txt`, as opposed to mirroring the explanation added in `runtime/doc/editing.txt`. I can mirror the explanation if needed, but keep in mind that `:h encryption` opens `runtime/doc/editing.txt` already.


Closes https://github.com/neovim/docs/pull/26
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