use make_weak_ref and resolve_ref from neutron-lib

The make_weak_ref and resolve_ref functions were rehomed into
neutron-lib with https://review.openstack.org/#/c/557809/
This patch consumes them by removing the functions and using lib's
version of them instead.

NeutronLibImpact

Change-Id: I41169ed0d93ea8caee530dcfc01a91271f02d538

neutron/common/utils.py

@@ -29,7 +29,6 @@ import sys
 import threading
 import time
 import uuid
-import weakref
 
 import eventlet
 from eventlet.green import subprocess
@@ -777,27 +776,6 @@ def get_related_rand_device_names(prefixes):
                                   max_length=n_const.DEVICE_NAME_MAX_LEN)
 
 
-try:
-    # PY3
-    weak_method = weakref.WeakMethod
-except AttributeError:
-    # PY2
-    import weakrefmethod
-    weak_method = weakrefmethod.WeakMethod
-
-
-def make_weak_ref(f):
-    """Make a weak reference to a function accounting for bound methods."""
-    return weak_method(f) if hasattr(f, '__self__') else weakref.ref(f)
-
-
-def resolve_ref(ref):
-    """Handles dereference of weakref."""
-    if isinstance(ref, weakref.ref):
-        ref = ref()
-    return ref
-
-
 def bytes_to_bits(value):
     return value * 8