CLN: remove IndexEngine.set_value
made possible bc Series._values now returns DTA/TDA for datetime64/timedelta64

Small perf improvement

```
In [3]: dti = pd.date_range('2016-01-01', freq='D', periods=10**4)              
In [4]: idx = list('abcdefghijklmnop')                                          
In [5]: arr = np.random.random(len(idx)*len(dti)).reshape(len(dti), -1)         
In [6]: df = pd.DataFrame(arr, index=dti, columns=idx)                          

In [7]: %timeit df2 = df._set_value("2043-05-14", "c", 4)                      
381 µs ± 14.3 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)   # <-- master
330 µs ± 13 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)  # <-- PR

```
ok this is fine