context: drop incorrect and superfluous docstring

It's been incorrect at least since 8b86acc7aa64 (context: drop support
for looking up context by ambiguous changeid (API), 2018-04-28).

Differential Revision: https://phab.mercurial-scm.org/D4880

mercurial/context.py

@@ -385,7 +385,6 b' class changectx(basectx):'
     changeset convenient. It represents a read-only context already present in
     the repo."""
     def __init__(self, repo, rev, node):
-        """changeid is a revision number, node, or tag"""
         super(changectx, self).__init__(repo)
         self._rev = rev
         self._node = node