Unused code for a `LetEnabled` REPL option

[brianhuffman]

The REPL implementation contains some code for enabling/disabling let declarations in user input. This includes the eLetEnabled field of the REPL RW type, and the functions enableLet, disableLet, and getLetEnabled in module Cryptol/REPL/Monad.hs. But apparently most of this is dead code, and the REPL provides no way to disable let expressions. (And why would you want to, anyway?)

Perhaps this LetEnabled code should all be removed.