[3.6] bpo-31285: Remove splitlines identifier from Python/_warnings.c (GH-3803) (#3829)

(forgot to remove it in GH-3219) (cherry picked from commit 8b4ff53)

Author: miss-islington

Python/_warnings.c

@@ -865,7 +865,6 @@ warnings_warn_explicit(PyObject *self, PyObject *args, PyObject *kwds)
 
  if (module_globals) {
  _Py_IDENTIFIER(get_source);
- _Py_IDENTIFIER(splitlines);
  PyObject *tmp;
  PyObject *loader;
  PyObject *module_name;
@@ -876,8 +875,6 @@ warnings_warn_explicit(PyObject *self, PyObject *args, PyObject *kwds)
 
  if ((tmp = _PyUnicode_FromId(&PyId_get_source)) == NULL)
  return NULL;
- if ((tmp = _PyUnicode_FromId(&PyId_splitlines)) == NULL)
- return NULL;
 
  /* Check/get the requisite pieces needed for the loader. */
  loader = PyDict_GetItemString(module_globals, "__loader__");