tools/gn: delete generate_test_gn_data tool

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 249 matching lines @@
         'value_unittest.cc',
         'visibility_unittest.cc',
       ],
       'dependencies': [
         'gn_lib',
         '../../base/base.gyp:run_all_unittests',
         '../../base/base.gyp:test_support_base',
         '../../testing/gtest.gyp:gtest',
       ],
     },
-    {
-      'target_name': 'generate_test_gn_data',
-      'type': 'executable',
-      'sources': [
-        'generate_test_gn_data.cc',
-      ],
-      'dependencies': [
-        '../../base/base.gyp:base',
-      ],
-    }
   ],
   'conditions': [
     ['test_isolation_mode != "noop"', {
       'targets': [
         {
           'target_name': 'gn_unittests_run',
           'type': 'none',
           'dependencies': [
             'gn_unittests',
           ],
           'includes': [ '../../build/isolate.gypi' ],
           'sources': [ 'gn_unittests.isolate' ],
         },
       ],
     }],
   ],
 }