Remove preconditions from ReadableStreamDefaultControllerError

ReadableStreamDefaultControllerError had a note that specifications should
respect the preconditions (listed as asserts). However, it doesn't have any
asserts. Remove mention of preconditions.

Fixes whatwg#920.

index.bs

@@ -1876,8 +1876,7 @@ export>ReadableStreamDefaultControllerError ( <var>controller</var>, <var>e</var
 
 This abstract operation can be called by other specifications that wish to move a readable stream to an errored state,
 in the same way a developer would error a stream using its associated controller object. Specifications should
-<em>not</em> do this to streams they did not create, and must ensure they have obeyed the precondition (listed here as
-an assert).
+<em>not</em> do this to streams they did not create.
 
 <emu-alg>
   1. Let _stream_ be _controller_.[[controlledReadableStream]].