DEPR: Deprecate Series.ravel

[phofl]

The methods looks quite unnecessary, the EAs are a no-op and for NumPy users can simply call ravel on the NumPy array.

[jorisvandenbossche]

+1 (I suppose this is mostly a left-over from Series subclassing np.ndarray initially (long time ago ;))

[jbrockmendel]

+1