exchange: drop unused '_getbookmarks' function

The function was introduced in 8491845a75b2 in mid-November 2016 but is never
used anywhere in core.

mercurial/exchange.py

@@ -1785,17 +1785,6 b' def _getbundletagsfnodes(bundler, repo, '
     outgoing = _computeoutgoing(repo, heads, common)
     bundle2.addparttagsfnodescache(repo, bundler, outgoing)
 
-def _getbookmarks(repo, **kwargs):
-    """Returns bookmark to node mapping.
-
-    This function is primarily used to generate `bookmarks` bundle2 part.
-    It is a separate function in order to make it easy to wrap it
-    in extensions. Passing `kwargs` to the function makes it easy to
-    add new parameters in extensions.
-    """
-
-    return dict(bookmod.listbinbookmarks(repo))
-
 def check_heads(repo, their_heads, context):
     """check if the heads of a repo have been modified