py3: unimplement RevlogError.__str__()

py3: unimplement RevlogError.__str__()

On Python 2, str(exc) would crash if __str__() returned a unicode containing
non-ASCII characters.

--- a/mercurial/error.py	Sun Dec 16 17:53:17 2018 +0900
+++ b/mercurial/error.py	Sun Dec 16 17:55:08 2018 +0900
@@ -44,14 +44,6 @@
 class RevlogError(StorageError):
     __bytes__ = _tobytes
 
-    def __str__(self):
-        # avoid cycle, and directly implement unimethod for this
-        # __str__ to allow delaying the import of encoding until
-        # someone actually wants the __str__ of a RevlogError (which
-        # should be very rare).
-        from . import encoding
-        return encoding.unifromlocal(_tobytes(self))
-
 class FilteredIndexError(IndexError):
     __bytes__ = _tobytes