Revert "NFC: utils/perf-training: Python 3 compatibility for lit.cfg"

This reverts commit 9178b10163f758cbf8a5290ea6a827990427ddc0 (r365969).

We are back to using Python2 and this is failing. This should instead be made
to be compatible with both Python 2 and 3.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@366953 91177308-0d34-0410-b5e6-96231b3b80d8

utils/perf-training/lit.cfg

@@ -10,7 +10,7 @@ def getSysrootFlagsOnDarwin(config, lit_config):
     # default system root path.
     if 'darwin' in config.target_triple:
         try:
-            out = subprocess.check_output(['xcrun', '--show-sdk-path']).strip().decode()
+            out = subprocess.check_output(['xcrun', '--show-sdk-path']).strip()
             res = 0
         except OSError:
             res = -1