lit: prefer discard over remove

Use `discard` rather than `remove` so that the exception is not raised
and we do not need to check if the key is present.

Author: compnerd

test/lit.cfg

@@ -1057,8 +1057,7 @@ elif run_os == 'linux-androideabi' or run_os == 'linux-android':
         tools_directory,
         '-L%s' % make_path(test_resource_dir, config.target_sdk_name)])
     # The Swift interpreter is not available when targeting Android.
-    if 'swift_interpreter' in config.available_features:
-        config.available_features.remove('swift_interpreter')
+    config.available_features.discard('swift_interpreter')
 
 else:
     lit_config.fatal("Don't know how to define target_run and "
@@ -1500,8 +1499,7 @@ else:
         config.available_features.add('libdispatch')
     else:
         # TSan runtime requires libdispatch on non-Apple platforms
-        if 'tsan_runtime' in config.available_features:
-            config.available_features.remove('tsan_runtime')
+        config.available_features.discard('tsan_runtime')
 
     if has_lib('Foundation'):
         config.available_features.add('foundation')