doc: Remove todo.txt

It's not documentation in the same sense as the majority of files in
runtime/doc, so is of little use to most users and should probably not
be installed alongside the "real" documentation.

It may be full of good ideas, but it's also full of things which are no
longer applicable to Nvim, such as references to platforms we no longer
support (e.g., MS-DOS), Vi compatibility, Autoconf, the Perl interface,
etc.

If someone is looking for things to fix or improve, the GitHub issue
tracker should prove useful, and, unlike todo.txt, the issues are
generally much more relevant to Nvim.  Besides all that, removing
todo.txt makes porting runtime patches a bit easier.

refs #2911, discussed briefly in #2608 and #2553

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