Org uses obsolete `filter-buffer-substring-functions` variable

[Stefan Kangas]

Org uses the obsolete variable `filter-buffer-substring-functions'.
This variable is likely due for deletion in Emacs 31.1.

Usually, compatibility code looks for the new way of doing things, and
then prefers that.  But it seems like someone decided here to look for
the old symbol instead, and prefer that.  Unless I'm missing something,
it seems to me that the code for the _new_ variable has been hitherto
unused.

Could you please take a look to confirm that removing
`filter-buffer-substring-functions' from Emacs won't break your code?
I have attached a diff.

Thanks!

filter-buffer-substring-functions.diff
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