Remove tonic/platform. (#16062)

This is unused.

Author: chinmaygarde

ci/licenses_golden/licenses_flutter

@@ -1168,9 +1168,6 @@ FILE: ../../../flutter/third_party/tonic/logging/dart_invoke.cc
 FILE: ../../../flutter/third_party/tonic/logging/dart_invoke.h
 FILE: ../../../flutter/third_party/tonic/parsers/packages_map.cc
 FILE: ../../../flutter/third_party/tonic/parsers/packages_map.h
-FILE: ../../../flutter/third_party/tonic/platform/platform_utils.h
-FILE: ../../../flutter/third_party/tonic/platform/platform_utils_posix.cc
-FILE: ../../../flutter/third_party/tonic/platform/platform_utils_win.cc
 FILE: ../../../flutter/third_party/tonic/scopes/dart_api_scope.h
 FILE: ../../../flutter/third_party/tonic/scopes/dart_isolate_scope.cc
 FILE: ../../../flutter/third_party/tonic/scopes/dart_isolate_scope.h

third_party/tonic/BUILD.gn

@@ -39,7 +39,6 @@ source_set("tonic") {
     "converter",
     "file_loader",
     "logging",
-    "platform",
     "scopes",
     "typed_data",
     "//third_party/dart/runtime:dart_api",

third_party/tonic/file_loader/BUILD.gn

@@ -25,7 +25,6 @@ source_set("file_loader") {
     "../converter",
     "../filesystem",
     "../parsers",
-    "../platform",
     "//third_party/dart/runtime:dart_api",
   ]
 }

third_party/tonic/file_loader/file_loader.cc

@@ -14,7 +14,6 @@
 #include "tonic/common/macros.h"
 #include "tonic/converter/dart_converter.h"
 #include "tonic/parsers/packages_map.h"
-#include "tonic/platform/platform_utils.h"
 
 namespace tonic {
 namespace {