runtime(doc): remove an obsolete item from todo.txt

runtime(doc): remove an obsolete item from todo.txt

I cannot reproduce this defect.  The replication instructions refer to
Mercurial version numbers and I no longer use Mercurial for my Vim
source.  I tried comparing various Git versions of the example file,
src/if_cscope.c, and still couldn't reproduce the defect.  Also, I use
Vim's diff mode a lot and haven't seen such a problem in a long time.
I am confident that it has been fixed.

Therefore, the item should be removed from todo.txt.

closes: #17534

Signed-off-by: Gary Johnson <garyjohn@spocom.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>