py: Remove unused and unneeded SystemError exception.

It's purpose is for internal errors that are not catastrophic (ie not as
bad as RuntimeError).  Since we don't use it, we don't need it.
diff --git a/py/objexcept.c b/py/objexcept.c
index a9c0436..203517b 100644
--- a/py/objexcept.c
+++ b/py/objexcept.c
@@ -268,7 +268,7 @@
       MP_DEFINE_EXCEPTION_BASE(IndentationError)
       MP_DEFINE_EXCEPTION(TabError, IndentationError)
       */
-  MP_DEFINE_EXCEPTION(SystemError, Exception)
+  //MP_DEFINE_EXCEPTION(SystemError, Exception)
   MP_DEFINE_EXCEPTION(TypeError, Exception)
   MP_DEFINE_EXCEPTION(ValueError, Exception)
     //TODO: Implement UnicodeErrors which take arguments