convert: drop a duplicate implementation of `dateutil.makedate()`

I noticed this because the signature generated by pytype recently changed to be
less specific.  When the method was introduced back in 337d728e644f,
`util.makedate()` didn't take an optional timestamp arg.  But now it does, and
the methods are the same (except the `dateutil` version validates that the
timestamp isn't a negative value).  I left the old method in place in case
anyone has custom convert code that monkey patches it.

hgext/convert/common.py

@@ -6,7 +6,6 b''
 # GNU General Public License version 2 or any later version.
 
 import base64
-import datetime
 import os
 import pickle
 import re
@@ -22,7 +21,10 b' from mercurial import ('
     pycompat,
     util,
 )
-from mercurial.utils import procutil
+from mercurial.utils import (
+    dateutil,
+    procutil,
+)
 
 propertycache = util.propertycache
 
@@ -565,12 +567,5 b' class mapfile(dict):'
             self.fp = None
 
 
-def makedatetimestamp(t):
-    """Like dateutil.makedate() but for time t instead of current time"""
-    tz = round(
-        t
-        - datetime.datetime.fromtimestamp(t)
-        .replace(tzinfo=datetime.timezone.utc)
-        .timestamp()
-    )
-    return t, tz
+def makedatetimestamp(t: float) -> dateutil.hgdate:
+    return dateutil.makedate(t)