Remove the libprotobuf_mutator BUILD (#4782)

Missed in #4731, noticed while looking at removing woff2

third_party/libprotobuf_mutator/BUILD.txt

@@ -1,26 +0,0 @@
-# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-# Exceptions. See /LICENSE for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-# libprotobuf_mutator uses cmake and doesn't provide a bazel BUILD file.
-# See https://github.com/google/libprotobuf-mutator/issues/91.
-
-load("@rules_cc//cc:defs.bzl", "cc_library")
-
-exports_files(["LICENSE"])
-
-cc_library(
-    name = "libprotobuf_mutator",
-    srcs = glob(
-        [
-            "src/**/*.cc",
-            "src/**/*.h",
-            "port/protobuf.h",
-        ],
-        exclude = ["**/*_test.cc"],
-    ),
-    hdrs = ["src/libfuzzer/libfuzzer_macro.h"],
-    strip_include_prefix = "src",
-    visibility = ["//visibility:public"],
-    deps = ["@protobuf"],
-)