Remove obsolete let

Firefox 35 compatibility problems:

As part of recent ES6 compliance efforts, the semantics of the `let` keyword have changed[1] in a non-backwards-compatible way which affects one or more of your add-ons. In particular:

1) Variables may no longer be declared more than once in the same scope using the `let` keyword, and may not be redeclared using the `let` keyword if they have already been declared using `var`, or as a function argument, within the same lexical scope. 

[1] https://bugzil.la/1001090