[lld][utils] Remove Python2 compatible imports in benchmark.py

These imports were moved around in Python 3.0
(https://docs.python.org/3/whatsnew/3.0.html#library-changes).

LLVM requires Python >= 3.8 so we can expect the Python3 names
to exist.

lld/utils/benchmark.py

@@ -13,13 +13,8 @@ import subprocess
 import json
 import datetime
 import argparse
-
-try:
-    from urllib.parse import urlencode
-    from urllib.request import urlopen, Request
-except ImportError:
-    from urllib import urlencode
-    from urllib2 import urlopen, Request
+from urllib.parse import urlencode
+from urllib.request import urlopen, Request
 
 
 parser = argparse.ArgumentParser()