remove redundant ref on HOLE

Author: namiroues

doc/source/installation.rst

@@ -257,8 +257,6 @@ MDAnalysisData_ is an additional package with datasets that can be used in examp
 
 This installation does not download all the datasets; instead, the datasets are cached when they are first downloaded using a Python command.
 
-
-.. _HOLE: http://www.holeprogram.org 
 .. _GCC: https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html
 .. _MDAnalysisData: https://www.mdanalysis.org/MDAnalysisData/
 .. _wiki: https://wiki.gentoo.org/wiki/GCC_optimization#-march