Remove DieIfOldLayerTreeWasNeverDiffed test

Author: knopp

ci/licenses_golden/licenses_flutter

@@ -36,7 +36,6 @@ FILE: ../../../flutter/flow/compositor_context.cc
 FILE: ../../../flutter/flow/compositor_context.h
 FILE: ../../../flutter/flow/diff_context.cc
 FILE: ../../../flutter/flow/diff_context.h
-FILE: ../../../flutter/flow/diff_context_unittests.cc
 FILE: ../../../flutter/flow/embedded_view_params_unittests.cc
 FILE: ../../../flutter/flow/embedded_views.cc
 FILE: ../../../flutter/flow/embedded_views.h

flow/BUILD.gn

@@ -137,7 +137,6 @@ if (enable_unittests) {
     testonly = true
 
     sources = [
-      "diff_context_unittests.cc",
       "embedded_view_params_unittests.cc",
       "flow_run_all_unittests.cc",
       "flow_test_utils.cc",