Remove dead option.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179670 91177308-0d34-0410-b5e6-96231b3b80d8

test/lit.cfg

@@ -235,10 +235,6 @@ if execute_external:
 if not platform.system() in ['Windows'] or lit.getBashPath() == '':
     config.available_features.add('shell-preserves-root')
 
-# For tests that require Darwin to run.
-if platform.system() in ['Darwin']:
-    config.available_features.add('system-darwin')
-
 # ANSI escape sequences in non-dumb terminal
 if platform.system() not in ['Windows']:
     config.available_features.add('ansi-escape-sequences')