[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
Just the tags, or the surrounding text as well?

On November 26, 2014 6:52:28 PM EST, "Justin M. Keyes" <notifications@github.com> wrote:
>> -	:setlocal cm=zip       " weak method, backwards compatible
>> -	:setlocal cm=blowfish  " strong method
>> -Do this before writing the file.  When reading an encrypted file it
>will be
>> -set automatically to the method used when that file was written. 
>You can
>> -change 'cryptmethod' before writing that file to change the method.
>> -To set the default method, used for new files, use one of these in
>your
>> -|vimrc| file: >
>> -	set cm=zip
>> -	set cm=blowfish
>> -The message given for reading and writing a file will show
>"[crypted]" when
>> -using zip, "[blowfish]" when using blowfish.
>> -
>> -When writing an undo file, the same key and method will be used for
>the text
>> -in the undo file. |persistent-undo|.
>> -
>> -						*E817* *E818* *E819* *E820*
>
>Let's keep these tags.
>
>---
>Reply to this email directly or view it on GitHub:
>https://github.com/neovim/neovim/pull/1563/files#r20971575