[LNT] Remove useless import fallback

Summary:
json module is available since Python 2.6. Since LNT now requires Python
2.7 or later, the fallback for importing simplejson in case json is not
available is useless. Removing it also removes one of the warning from
mypy.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, leandron, PrzemekWirkus

Reviewed By: cmatthews

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D69054

Author: Thomas Preud'homme

lnt/testing/__init__.py

@@ -7,14 +7,10 @@
 """
 
 import datetime
+import json
 import re
 from lnt.util import logger
 
-try:
-    import json
-except ImportError:
-    import simplejson as json
-
 # We define the following constants for use as sample values by
 # convention.
 PASS = 0