Issue 18719: Remove a false optimization

Remove an unused early-out test from the critical path for
dict and set lookups.

When the strings already have matching lengths and hashes,
there is no additional information gained by checking the first
characters (the probability of a mismatch is already known to
be less than 1 in 2**64).

Objects/stringobject.c

@@ -1255,7 +1255,6 @@ _PyString_Eq(PyObject *o1, PyObject *o2)
     PyStringObject *a = (PyStringObject*) o1;
     PyStringObject *b = (PyStringObject*) o2;
     return Py_SIZE(a) == Py_SIZE(b)
-      && *a->ob_sval == *b->ob_sval
       && memcmp(a->ob_sval, b->ob_sval, Py_SIZE(a)) == 0;
 }