Remove old indentation and navigation code on octave-mode.

Remove old indentation and navigation code on octave-mode.
* lisp/progmodes/octave-mod.el (octave-mode-map): Remap down-list to
smie-down-list rather than add a binding for octave-down-block.
(octave-mark-block, octave-blink-matching-block-open):
Rely on forward-sexp-function.
(octave-fill-paragraph): Don't narrow, so you can use
indent-according-to-mode.
(octave-block-begin-regexp, octave-block-begin-or-end-regexp): Remove.
(octave-in-block-p, octave-re-search-forward-kw)
(octave-re-search-backward-kw, octave-indent-calculate)
(octave-end-as-array-index-p, octave-block-end-offset)
(octave-scan-blocks, octave-forward-block, octave-backward-block)
(octave-down-block, octave-backward-up-block, octave-up-block)
(octave-before-magic-comment-p, octave-indent-line): Remove.