obsstore: drop 'date' from the metadata dictionary

We are extracting the `date` information from the metadata at read time.
However, we failed to remove it from the metadata returned in the markers. This
is now fixed.

mercurial/obsolete.py

@@ -168,10 +168,12 b' def _readmarkers(data):'
                                'short, %d bytes expected, got %d')
                              % (mdsize, len(metadata)))
         off += mdsize
+        meta = decodemeta(metadata)
         try:
             date = util.parsedate(decodemeta(metadata).pop('date', '0 0'))
         except util.Abort:
             date = (0., 0)
+        metadata = encodemeta(meta)
 
         yield (pre, sucs, flags, metadata, date, None)