Remove unused context manager

cbor2/decoder.py

@@ -1,7 +1,6 @@
 import re
 import struct
 from datetime import datetime, timedelta
-from contextlib import contextmanager
 from io import BytesIO
 
 from .compat import timezone, xrange, byte_as_integer, unpack_float16
@@ -418,14 +417,6 @@ def decode_from_bytes(self, buf):
         self.fp = old_fp
         return retval
 
-    @contextmanager
-    def key_decoder(self):
-        """ Forces decoders that return mutable types by default to produce immutable types."""
-        original_flag = self._immutable
-        self._immutable = True
-        yield self.decode
-        self._immutable = original_flag
-
 
 def loads(payload, **kwargs):
     """