Remove duplicate and outdated src/third_party/dart/tools/sdks entry from DEPS. (flutter#14340)

* Drop duplicate entry

Author: aam

DEPS

@@ -494,16 +494,6 @@ deps = {
      'dep_type': 'cipd',
    },
 
-   'src/third_party/dart/tools/sdks': {
-     'packages': [
-       {
-         'package': 'dart/dart-sdk/${{platform}}',
-         'version': 'version:2.4.0'
-       }
-     ],
-     'dep_type': 'cipd',
-   },
-
    'src/third_party/dart/pkg/analysis_server/language_model': {
      'packages': [
        {