Revert "ui/ozone: runtime-map "gbm" platform to "drm""

This reverts commit 64b5f971.

With https://crrev.com/c/2423085 merged, our tests are not using the
"drm" option anymore, so we can remove this temporary map.

BUG=b:169115289
BUG=1116466
TEST=video.EncodeAccel.h264_360p_i420 passes on Kukui.

Change-Id: Ifc7b8fe6b1b588f18bd26ba38b117ead6f172c12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437311Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813076}

ui/ozone/platform_selection.cc

@@ -33,9 +33,6 @@ int GetOzonePlatformId() {
     return g_selected_platform;
 
   std::string platform_name = GetPlatformName();
-  // TODO(b/169115289) remove once all Tast tests use "drm".
-  if (platform_name == "gbm")
-    platform_name = "drm";
 
   // Search for a matching platform in the list.
   for (int platform_id = 0; platform_id < kPlatformCount; ++platform_id) {