Remove unused benchmark_main.h (#4409)

This was refactored to Testing::GetExePath, but apparently the header
was missed.

common/benchmark_main.h

@@ -1,22 +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
-
-#ifndef CARBON_COMMON_BENCHMARK_MAIN_H_
-#define CARBON_COMMON_BENCHMARK_MAIN_H_
-
-#include "llvm/ADT/StringRef.h"
-
-// When using the Carbon `main` function for benchmarks, we export some extra
-// information about the test binary that can be accessed with this header.
-//
-// TODO: Refactor this to share code with `gtest_main.h`.
-
-namespace Carbon::Testing {
-
-// Returns the executable path of the benchmark binary.
-auto GetBenchmarkExePath() -> llvm::StringRef;
-
-}  // namespace Carbon::Testing
-
-#endif  // CARBON_COMMON_BENCHMARK_MAIN_H_