gh-140494: Remove dead code ifndef TCL_THREADS, which is always defined

[mdehoon]

Modules/_tkinter.c always defines TCL_THREADS:

/* If Tcl is compiled for threads, we must also define TCL_THREAD. We define
   it always; if Tcl is not threaded, the thread functions in
   Tcl are empty.  */ 
#define TCL_THREADS

But in two places in Modules/_tkinter.c, we check if TCL_THREADS is defined.

#ifdef TCL_THREADS
static Tcl_ThreadDataKey state_key;
typedef PyThreadState *ThreadSpecificData;
#define tcl_tstate \
    (*(PyThreadState**)Tcl_GetThreadData(&state_key, sizeof(PyThreadState*)))
#else
static PyThreadState *tcl_tstate = NULL;
#endif

and

#ifndef TCL_THREADS
    if (v->threaded) {
        PyErr_SetString(PyExc_RuntimeError,
                        "Tcl is threaded but _tkinter is not");
        Py_DECREF(v);
        return 0;
    }
#endif

This PR is a minimal patch that removes the dead code, updates one comment, and fixes one typo in a comment.

• Issue: Dead code in tkinter #140494

[serhiy-storchaka]

LGTM, but hold until we backport what is backportable.