Remove unused file picture_serializer. (flutter#6179)

Author: chinmaygarde

ci/licenses_golden/licenses_flutter

@@ -819,8 +819,6 @@ FILE: ../../../flutter/shell/common/animator.cc
 FILE: ../../../flutter/shell/common/animator.h
 FILE: ../../../flutter/shell/common/engine.cc
 FILE: ../../../flutter/shell/common/engine.h
-FILE: ../../../flutter/shell/common/picture_serializer.cc
-FILE: ../../../flutter/shell/common/picture_serializer.h
 FILE: ../../../flutter/shell/common/platform_view.cc
 FILE: ../../../flutter/shell/common/platform_view.h
 FILE: ../../../flutter/shell/common/rasterizer.cc

shell/common/BUILD.gn

@@ -67,8 +67,6 @@ source_set("common") {
     "io_manager.h",
     "isolate_configuration.cc",
     "isolate_configuration.h",
-    "picture_serializer.cc",
-    "picture_serializer.h",
     "platform_view.cc",
     "platform_view.h",
     "rasterizer.cc",