Remove old and unused test file (#589)

This file was needed in older versions of Dart. It's currently not used,
and the tests do not pass when run, probably because of the static
variable in protoc_plugin/lib/src/code_generator.dart:23.
diff --git a/protoc_plugin/test/all_tests.dart b/protoc_plugin/test/all_tests.dart
deleted file mode 100755
index d85093f..0000000
--- a/protoc_plugin/test/all_tests.dart
+++ /dev/null
@@ -1,90 +0,0 @@
-#!/usr/bin/env dart
-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// @dart=2.11
-
-import 'any_test.dart' as any;
-import 'bazel_test.dart' as bazel;
-import 'client_generator_test.dart' as client_generator;
-import 'const_generator_test.dart' as const_generator;
-import 'default_value_escape_test.dart' as default_value_escape;
-import 'descriptor_test.dart' as descriptor_test;
-import 'enum_generator_test.dart' as enum_generator;
-import 'extension_generator_test.dart' as extension_generator_test;
-import 'extension_test.dart' as extension_test;
-import 'file_generator_test.dart' as file_generator;
-import 'generated_message_test.dart' as generated_message;
-import 'hash_code_test.dart' as hash_code;
-import 'high_tagnumber_test.dart' as high_tagnumber;
-import 'import_public_test.dart' as import_public;
-import 'import_test.dart' as import_prefix;
-import 'indenting_writer_test.dart' as indenting_writer;
-import 'json_test.dart' as json;
-import 'leading_underscores_test.dart' as leading_underscores;
-import 'map_field_test.dart' as map_field;
-import 'map_test.dart' as map;
-import 'merge_test.dart' as merge;
-import 'message_generator_test.dart' as message_generator;
-import 'message_test.dart' as message;
-import 'mixin_test.dart' as mixin_test;
-import 'names_test.dart' as names;
-import 'omit_enum_names_test.dart' as omit_enum_names;
-import 'omit_field_names_test.dart' as omit_field_names;
-import 'omit_message_names_test.dart' as omit_message_names;
-import 'oneof_test.dart' as oneof;
-import 'proto3_optional_test.dart' as proto3_optional;
-import 'protoc_options_test.dart' as protoc_options;
-import 'repeated_field_test.dart' as repeated_field;
-import 'reserved_names_test.dart' as reserved_names;
-import 'send_protos_via_sendports_test.dart' as send_protos_via_sendports_test;
-import 'service_generator_test.dart' as service_generator;
-import 'service_test.dart' as service;
-import 'to_builder_test.dart' as to_builder;
-import 'unknown_field_set_test.dart' as unknown_field_set;
-import 'validate_fail_test.dart' as validate_fail;
-import 'wire_format_test.dart' as wire_format;
-
-void main() {
-  any.main();
-  bazel.main();
-  client_generator.main();
-  const_generator.main();
-  default_value_escape.main();
-  descriptor_test.main();
-  enum_generator.main();
-  extension_generator_test.main();
-  extension_test.main();
-  file_generator.main();
-  generated_message.main();
-  hash_code.main();
-  high_tagnumber.main();
-  import_prefix.main();
-  import_public.main();
-  indenting_writer.main();
-  json.main();
-  leading_underscores.main();
-  map.main();
-  map_field.main();
-  merge.main();
-  message.main();
-  message_generator.main();
-  mixin_test.main();
-  names.main();
-  omit_enum_names.main();
-  omit_field_names.main();
-  omit_message_names.main();
-  oneof.main();
-  proto3_optional.main();
-  protoc_options.main();
-  repeated_field.main();
-  reserved_names.main();
-  send_protos_via_sendports_test.main();
-  service.main();
-  service_generator.main();
-  to_builder.main();
-  unknown_field_set.main();
-  validate_fail.main();
-  wire_format.main();
-}