Drop pylsim docs link

Author: jeromekelleher

docs/_config.yml

@@ -65,7 +65,6 @@ sphinx:
         python: ["https://docs.python.org/3/", null]
         tutorials: ["https://tskit.dev/tutorials/", null]
         stdpopsim: ["https://stdpopsim.readthedocs.io/en/stable", null]
-        pyslim: ["https://tskit.dev/pyslim/docs/latest/", null]
         msprime: ["https://tskit.dev/msprime/docs/stable/", null]
         numpy: ["https://numpy.org/doc/stable/", null]