CLN: remove unused Series._index attribute
Noticed this in https://github.com/pandas-dev/pandas/pull/42950, the attribute seems completely unused. 

(cc @jbrockmendel do you have any idea what the reason is/was for this?)
> do you have any idea what the reason is/was for this?)

no idea yep, no idea what this was used for, thanks @jorisvandenbossche 