interfaces: drop the unused `ipeerv2`

I couldn't find a reference to `apidescriptor` anywhere, and it turns out the
rest of the code added with this back in dac438b7346e was abandoned and removed
in 04688c51f81f.

mercurial/interfaces/repository.py

@@ -390,14 +390,6 class ipeerbase(ipeerconnection, ipeerca
     """
 
 
-class ipeerv2(ipeerconnection, ipeercapabilities, ipeerrequests):
-    """Unified peer interface for wire protocol version 2 peers."""
-
-    apidescriptor = interfaceutil.Attribute(
-        """Data structure holding description of server API."""
-    )
-
-
 @interfaceutil.implementer(ipeerbase)
 class peer:
     """Base class for peer repositories."""