orientation sensor: Remove unused variable from test

kRotationDOMMatrix has not been used since commit 7e8c0421 ("[sensors] Fix
orientation sensor layout test").

Change-Id: I2c85083df36e778acc0512bb8212a18f564334d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742132
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685256}

third_party/blink/web_tests/sensor/orientation-sensor.html

@@ -18,7 +18,6 @@ const kRotationMatrix = [1,  0,  0,  0,
                          0, -1,  0,  0,
                          0,  0, -1,  0,
                          0,  0,  0,  1];
-const kRotationDOMMatrix = new DOMMatrix(kRotationMatrix);
 
 // For 'orientation.angle == 270', which is set for tests at
 // at SensorProxy::GetScreenOrientationAngle().