Revert "Enable UOp optimizer by default in main interpreter"

This reverts commit 7db1d12.

Author: gvanrossum

Python/pylifecycle.c

@@ -1180,11 +1180,6 @@ init_interp_main(PyThreadState *tstate)
 #endif
     }
 
-    if (is_main_interp) {
-        PyObject *opt = PyUnstable_Optimizer_NewUOpOptimizer();
-        PyUnstable_SetOptimizer((_PyOptimizerObject *)opt);
-    }
-
     assert(!_PyErr_Occurred(tstate));
 
     return _PyStatus_OK();