Pass the flag instead of using a global. (#893)

Arguably missed in #769 

Note, this is reminding me we have more class members to rename for `_`, but I felt it's best to use the new naming instead of adding more to clean up.

executable_semantics/common/BUILD

@@ -38,10 +38,3 @@ cc_library(
         "@llvm-project//llvm:Support",
     ],
 )
-
-cc_library(
-    name = "tracing_flag",
-    srcs = ["tracing_flag.cpp"],
-    hdrs = ["tracing_flag.h"],
-    visibility = ["//executable_semantics:__subpackages__"],
-)

executable_semantics/common/tracing_flag.cpp

@@ -1,11 +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
-
-#include "executable_semantics/common/tracing_flag.h"
-
-namespace Carbon {
-
-bool tracing_output = false;
-
-}  // namespace Carbon

executable_semantics/common/tracing_flag.h

@@ -1,15 +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 EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_
-#define EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_
-
-namespace Carbon {
-
-// Program option to enable/disable tracing.
-extern bool tracing_output;
-
-}  // namespace Carbon
-
-#endif  // EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_