Misc code cleanup of octave-mode.el.

Misc code cleanup of octave-mode.el.
* lisp/progmodes/octave-mod.el (octave-mode-map): Remove special bindings
for (un)commenting the region and performing completion.
(octave-mode-menu): Use standard commands for help and completion.
(octave-mode-syntax-table): Support %{..%} comments (sort of).
(octave-mode): Use define-derived-mode.
Set completion-at-point-functions and don't set columns.
Don't disable adaptive-fill-regexp.
(octave-describe-major-mode, octave-comment-region)
(octave-uncomment-region, octave-comment-indent)
(octave-indent-for-comment): Remove.
(octave-indent-calculate): Rename from calculate-octave-indent.
(octave-indent-line): Update caller.
(octave-initialize-completions): No need to make an alist.
(octave-completion-at-point-function): New function.
(octave-complete-symbol): Use it.
(octave-insert-defun): Use define-skeleton.