Remove redundant check in TransformStreamDefaultSinkCloseAlgorithm

Author: MattiasBuelens

index.bs

@@ -5310,9 +5310,7 @@ side=] of [=transform streams=].
  1. Return the result of [=reacting=] to |flushPromise|:
   1. If |flushPromise| was fulfilled, then:
    1. If |readable|.\[[state]] is "`errored`", throw |readable|.\[[storedError]].
-   1. Let |readableController| be |readable|.\[[readableStreamController]].
-   1. If ! [$ReadableStreamDefaultControllerCanCloseOrEnqueue$](|readableController|) is true,
-      perform ! [$ReadableStreamDefaultControllerClose$](|readableController|).
+   1. Perform ! [$ReadableStreamDefaultControllerClose$](|readable|.\[[readableStreamController]]).
   1. If |flushPromise| was rejected with reason |r|, then:
    1. Perform ! [$TransformStreamError$](|stream|, |r|).
    1. Throw |readable|.\[[storedError]].

reference-implementation/lib/abstract-ops/transform-streams.js

@@ -244,10 +244,7 @@ function TransformStreamDefaultSinkCloseAlgorithm(stream) {
     if (readable._state === 'errored') {
       throw readable._storedError;
     }
-    const readableController = readable._readableStreamController;
-    if (ReadableStreamDefaultControllerCanCloseOrEnqueue(readableController) === true) {
-      ReadableStreamDefaultControllerClose(readableController);
-    }
+    ReadableStreamDefaultControllerClose(readable._readableStreamController);
   }, r => {
     TransformStreamError(stream, r);
     throw readable._storedError;