[issue1771] Remove cmp parameter to list.sort() and builtin.sorted()

[Lea Wiemann]

Lea Wiemann added the comment:

"Non-obvious" to the novice that this technique can be used, and non-
obvious to the reader of the program.

I'm envisioning key functions that return long sequences of -1/0/1 
integers, or numbers between 0 and 2**50...  Not good.

Instead of removing cmp, I'd suggest simply placing a note in the 
documentation saying that key is preferred over cmp, to encourage 
readability.

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