localrepo: delete obsolete comment about `prefix in repo` raising exception

We dropped support for `prefix in repo`, where `prefix` is a hex
prefix, in 8b86acc7aa64 (context: drop support for looking up context
by ambiguous changeid (API), 2018-04-28), after having deprecated it a
while before that. These days you get a `ProgrammingError` instead if
you pass in a short nodeid.

Differential Revision: https://phab.mercurial-scm.org/D9599

mercurial/localrepo.py

@@ -1754,11 +1754,7 b' class localrepository(object):'
             return context.workingctx(self)
 
     def __contains__(self, changeid):
-        """True if the given changeid exists
-
-        error.AmbiguousPrefixLookupError is raised if an ambiguous node
-        specified.
-        """
+        """True if the given changeid exists"""
         try:
             self[changeid]
             return True