Add back in_c_key and remove old docstring paragraph.

theano/configdefaults.py

@@ -42,7 +42,8 @@ AddConfigVar('floatX',
              "Note: float16 support is experimental, use at your own risk.",
              EnumStr('float64', 'float32', 'float16',
                      convert=floatX_convert,),
-             in_c_key=False
+             # TODO: see gh-4466 for how to remove it.
+             in_c_key=True
              )
 
 AddConfigVar('warn_float64',

theano/gof/cc.py

@@ -1229,10 +1229,6 @@ class CLinker(link.Linker):
         It is followed by elements for every node in the topological ordering
         of `self.fgraph`.
 
-        If the Op of any Apply in the FunctionGraph does not have
-        c_code_cache_ok()==True, then this function raises a KeyError
-        exception.
-
         Input Signature
         ---------------