gh-142873: Do not check for PyContextVar_CheckExact twice in PyContextVar_Set

Author: sobolevn

Python/context.c

@@ -1019,8 +1019,6 @@ static PyObject *
 _contextvars_ContextVar_get_impl(PyContextVar *self, PyObject *default_value)
 /*[clinic end generated code: output=0746bd0aa2ced7bf input=da66664d5d0af4ad]*/
 {
-    ENSURE_ContextVar(self, NULL)
-
     PyObject *val;
     if (PyContextVar_Get((PyObject *)self, default_value, &val) < 0) {
         return NULL;