remove unused link definition

Author: rouilj

doc/installation.txt

@@ -1738,7 +1738,6 @@ Once you've unpacked roundup's source, if you have pytest installed,
 run ``python -m pytest test`` in the source directory and make sure
 there are no errors. If there are errors, please let us know!
 
-.. _`table of contents`: index.html
 .. _`user guide`: user_guide.html
 .. _`roundup specification`: spec.html
 .. _`tracker configuration`: customizing.html#tracker-configuration