[ozone/wayland] Remove duplicated DCHECK.

This change doesn't bring any functionality changes, but rather
removes an accidentely added duplicate DCHECK.

TBR=rjkroege@chromium.org

Bug: 784836, 578890
Change-Id: I524178327c4f6131eb582b9de37afe799039a41e
Reviewed-on: https://chromium-review.googlesource.com/1051226
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#557116}

ui/ozone/platform/wayland/wayland_window.cc

@@ -193,8 +193,6 @@ void WaylandWindow::Maximize() {
 }
 
 void WaylandWindow::Minimize() {
-  DCHECK(xdg_surface_);
-
   DCHECK(xdg_surface_);
   xdg_surface_->SetMinimized();
   connection_->ScheduleFlush();