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It may be full of good ideas, but it's also full of things which are no
longer applicable to Nvim, such as references to platforms we no longer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

runtime/doc/gui_w32.txt

@@ -243,7 +243,7 @@ selection.  You can use CTRL-Q instead.  You can also use CTRL-Q in Insert
 mode and Command-line mode to get the old meaning of CTRL-V.  But CTRL-Q
 doesn't work for terminals when it's used for control flow.
 
-NOTE: The clipboard support still has a number of bugs.  See |todo|.
+NOTE: The clipboard support still has a number of bugs.
 
 ==============================================================================
 4. Shell Commands					*gui-shell-win32*

runtime/doc/help.txt

@@ -94,7 +94,6 @@ General subjects ~
 |tips.txt|	various tips on using Vim
 |message.txt|	(error) messages and explanations
 |quotes.txt|	remarks from users of Vim
-|todo.txt|	known problems and desired extensions
 |develop.txt|	development of Vim
 |debug.txt|	debugging Vim itself
 |uganda.txt|	Vim distribution conditions and what to do with your money