Remove LayerBuilder and DefaultLayerBuilder (flutter#6256)

This essentially reverts flutter#4197
as no one is (or soon will be) implementing an alternative LayerBuilder.
Let's just put everything in SceneBuilder to reduce the YAGNI
(you aren't gonna need it) smell. This will also make retained rendering
API changes much easier.

ci/licenses_golden/licenses_flutter

@@ -248,10 +248,6 @@ FILE: ../../../flutter/assets/asset_manager.h
 FILE: ../../../flutter/assets/asset_resolver.h
 FILE: ../../../flutter/common/task_runners.cc
 FILE: ../../../flutter/common/task_runners.h
-FILE: ../../../flutter/flow/layers/default_layer_builder.cc
-FILE: ../../../flutter/flow/layers/default_layer_builder.h
-FILE: ../../../flutter/flow/layers/layer_builder.cc
-FILE: ../../../flutter/flow/layers/layer_builder.h
 FILE: ../../../flutter/flow/skia_gpu_object.cc
 FILE: ../../../flutter/flow/skia_gpu_object.h
 FILE: ../../../flutter/runtime/dart_isolate.cc

flow/BUILD.gn

@@ -22,12 +22,8 @@ source_set("flow") {
     "layers/color_filter_layer.h",
     "layers/container_layer.cc",
     "layers/container_layer.h",
-    "layers/default_layer_builder.cc",
-    "layers/default_layer_builder.h",
     "layers/layer.cc",
     "layers/layer.h",
-    "layers/layer_builder.cc",
-    "layers/layer_builder.h",
     "layers/layer_tree.cc",
     "layers/layer_tree.h",
     "layers/opacity_layer.cc",