Fix the issue of before-save-hook being added globally

[akirak]

When you enable org-make-toc-mode, org-make-toc is added to before-save-hook globally, and the following message is printed every time if you save a buffer, even if the buffer is not in org-mode:

org-make-toc: No TOC node found.

This PR fixes the issue.

[alphapapa]

Hi Akira,

Thanks for sending this fix. Apparently I overlooked it in my notifications; feel free to ping me in the future if I miss such things. This is a simple, obvious fix, but I actually don't use the mode myself (I add to the before-save-hook in specific files' local variables), so I hadn't noticed it.

[akirak]

I see. I have been using this patch for two years myself. I rebased the branch just to update my installation yesterday. No worries.

Nowadays GitHub has built-in TOC navigation, so generating TOCs doesn't seem obligatory.