Attempt to fix TSan issues in Swift unit tests.

The only reason we are messing with the delegate's fields is because we had no
remove API, but we have that now.  And maybe this will fix the claimed thread
race between us setting its fields and it being destroyed.

Author: bzbarsky-apple

src/darwin/Framework/CHIPTests/MTRSwiftDeviceTests.swift

@@ -414,12 +414,8 @@ class MTRSwiftDeviceTests : XCTestCase {
 
         wait(for: [ resubscriptionReachableExpectation, resubscriptionGotReportsExpectation ], timeout:60)
 
-        // Now make sure we ignore later tests.  Ideally we would just unsubscribe
-        // or remove the delegate, but there's no good way to do that.
-        delegate.onReachable = { () -> Void in }
-        delegate.onNotReachable = nil
-        delegate.onAttributeDataReceived = nil
-        delegate.onEventDataReceived = nil
+        // Now make sure we ignore later tests.
+        device.remove(_: delegate)
 
         // Make sure we got no updated reports (because we had a cluster state cache
         // with data versions) during the resubscribe.