Remove unused lexer code.

mojom/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 group("mojom") {
   testonly = true
-  deps = [
-    ":tests",
-  ]
   if (is_linux && defined(go_build_tool) && go_build_tool != "") {
-    deps += [ "//mojom/mojom_parser" ]
+    deps = [
+      "//mojom/mojom_parser",
+    ]
   }
 }
-
-group("tests") {
-  testonly = true
-  deps = [
-    ":lexer_unittest",
-  ]
-}
-
-test("lexer_unittest") {
-  sources = [
-    "lexer_unittest.cc",
-  ]
-
-  deps = [
-    "//base",
-    "//base/test:run_all_unittests",
-    "//testing/gtest",
-    ":lexer",
-  ]
-}
-
-source_set("lexer") {
-  sources = [
-    "lexer.cc",
-    "lexer.h",
-  ]
-
-  deps = [
-    "//base",
-  ]
-}