Simplify lexical let-optimisations

Simplify lexical let-optimisations

Ensure in cconv that let-bindings have the normal form (VAR EXPR)
where VAR is a valid variable name, so that we don't need to keep
re-checking this all the time in the optimiser.

* lisp/emacs-lisp/byte-opt.el
(byte-optimize-enable-variable-constprop)
(byte-optimize-warn-eliminated-variable): Remove; these were mainly
used for debugging.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form):
Assume normalised let-bindings (with lexical-binding).
Stop using the variables removed above.
* lisp/emacs-lisp/cconv.el (cconv-convert): Ensure normalised
let-bindings.  Malformed bindings are dropped after warning.

remove byte-optimize-warn-eliminated-variable