[NFC] Remove Python 2 imports from __future__ (#42086)

The `__future__` we relied on is now,  where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):

* absolute_import
* print_function
* unicode_literals
* division

These import statements are no-ops and are no longer necessary.

Author: dduan

benchmark/scripts/Benchmark_DTrace.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # ===--- Benchmark_DTrace.in ---------------------------------------------===//
 #

benchmark/scripts/Benchmark_Driver

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 # ===--- Benchmark_Driver ------------------------------------------------===//

benchmark/scripts/Benchmark_GuardMalloc.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # ===--- Benchmark_GuardMalloc.in ----------------------------------------===//
 #

benchmark/scripts/Benchmark_QuickCheck.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # ===--- Benchmark_QuickCheck.in -----------------------------------------===//
 #

benchmark/scripts/Benchmark_RuntimeLeaksRunner.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # ===--- Benchmark_RuntimeLeaksRunner.in ---------------------------------===//
 #

benchmark/scripts/build_linux.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import argparse
 import os

benchmark/scripts/build_script_helper.py

@@ -1,6 +1,4 @@
-#!/usr/bin/env python
-
-from __future__ import print_function
+#!/usr/bin/env python3
 
 import argparse
 import os

benchmark/scripts/compare_perf_tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 # ===--- compare_perf_tests.py -------------------------------------------===//
@@ -27,8 +27,6 @@ class `ReportFormatter` creates the test comparison report in specified format.
 
 """
 
-from __future__ import print_function
-
 import argparse
 import functools
 import re

benchmark/scripts/create_benchmark.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import argparse
 import os

benchmark/scripts/generate_harness/generate_harness.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # ===--- generate_harness.py ---------------------------------------------===//
 #
@@ -14,8 +14,6 @@
 
 # Generate boilerplate, CMakeLists.txt and utils/main.swift from templates.
 
-from __future__ import print_function
-
 import argparse
 import os
 import subprocess