PAS: drop NullStdoutMixin

Ref #958.

For some reason, fpc crashes during compile errors if this is specified,
despite never writing to stdout if it isn't.

dmoj/executors/PAS.py

@@ -3,7 +3,7 @@
 from dmoj.executors.mixins import NullStdoutMixin
 
 
-class Executor(NullStdoutMixin, CompiledExecutor):
+class Executor(CompiledExecutor):
     ext = 'pas'
     name = 'PAS'
     command = 'fpc'