don't update mtime when moving

This is incorrect when the file was out-of-sync when moved. A possible approach
in the future could check whether the old mtime was up to date and, in that case
only, keep it up to date with the new filename.

beets/library.py

@@ -234,9 +234,6 @@ class Item(object):
         # Either copying or moving succeeded, so update the stored path.
         self.path = dest
 
-        # Update mtime to reflect the new file.
-        self.mtime = self.current_mtime()
-
     def current_mtime(self):
         """Returns the current mtime of the file, rounded to the nearest
         integer.