Remove workaround code that is not needed since #1211 (#2683)

include/pybind11/pybind11.h

@@ -2089,15 +2089,7 @@ public:
         }
 
         if (release) {
-            /* Work around an annoying assertion in PyThreadState_Swap */
-            #if defined(Py_DEBUG)
-                PyInterpreterState *interp = tstate->interp;
-                tstate->interp = nullptr;
-            #endif
             PyEval_AcquireThread(tstate);
-            #if defined(Py_DEBUG)
-                tstate->interp = interp;
-            #endif
         }
 
         inc_ref();