mergemod: drop support for merge.update without a target

This was to be deleted after 3.9.

mercurial/merge.py

@@ -25,7 +25,6 b' from .node import ('
 )
 from . import (
     copies,
-    destutil,
     error,
     filemerge,
     obsolete,
@@ -1484,11 +1483,6 b' def update(repo, node, branchmerge, forc'
         if ancestor is not None:
             pas = [repo[ancestor]]
 
-        if node is None:
-            repo.ui.deprecwarn('update with no target', '3.9')
-            rev, _mark, _act = destutil.destupdate(repo)
-            node = repo[rev].node()
-
         overwrite = force and not branchmerge
 
         p2 = repo[node]