Remove discard_reason.h from chrome sources lists

The file discard_reason.h was deleted when its content
moved into a mojom file in
https://chromium-review.googlesource.com/1176267 but
it was not removed from the sources lists. This caused
some local problems with trybot servers so here it goes.

Change-Id: I873bc5e643c87f83197b4f0512d2859371c1e005
Reviewed-on: https://chromium-review.googlesource.com/c/1288794
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601065}

chrome/browser/BUILD.gn

@@ -2841,7 +2841,6 @@ jumbo_split_static_library("browser") {
       "resource_coordinator/decision_details.h",
       "resource_coordinator/discard_metrics_lifecycle_unit_observer.cc",
       "resource_coordinator/discard_metrics_lifecycle_unit_observer.h",
-      "resource_coordinator/discard_reason.h",
       "resource_coordinator/intervention_policy_database.cc",
       "resource_coordinator/intervention_policy_database.h",
       "resource_coordinator/leveldb_site_characteristics_database.cc",