No ticket: Refactoring in Effects.js. Using data_priv.

I realized line 307 of Effects.js:
`dataShow = jQuery._data( elem, "fxshow" ) || jQuery._data( elem, "fxshow", {} );`
was not changed to use data_priv.

I made that modification myself, using `data_priv.get`. And added `dataShow !== undefined` in line 316, because it is possible for the `data_priv.get` to return an undefined object. This case happens in test 6 (  animate(Hash, Object, Function)  ) of Effects without that check.

Also, there is no more reason for the `if ( !length )` check in line 327. I was using it to destroy the object I was creating in line 307, a cleanup. It was a `jQuery._removeData( elem, "fxshow" );` when the data_priv change occurred, there was probably a merge conflict, so it was changed that to `data_priv.get` and `data_priv.access` in the proccess. We do not need it anymore anyway.