[ozone/drm/mojo] Remove a mash only DCHECK

Ozone/drm/mojo contained a DCHECK only appropriate for legacy mash mode.
This mode has been removed so remove the DCHECK as well.

BUG=620927
TEST=by hand

Change-Id: Ieb8c6b21ed79de2202ee06eedf47029d32183421
Reviewed-on: https://chromium-review.googlesource.com/1153968Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578859}

ui/ozone/platform/drm/host/drm_device_connector.cc

@@ -42,9 +42,6 @@ DrmDeviceConnector::DrmDeviceConnector(
       ws_runner_(base::ThreadTaskRunnerHandle::IsSet()
                      ? base::ThreadTaskRunnerHandle::Get()
                      : nullptr) {
-  // Invariant: we only have a runner at startup if executing in mash mode.
-  DCHECK((ws_runner_ && !features::IsAshInBrowserProcess()) ||
-         (!ws_runner_ && features::IsAshInBrowserProcess()));
 }
 
 DrmDeviceConnector::~DrmDeviceConnector() {}