data: Merge dupe local and data_ variables.

Follows-up b9cdc4136b688963d1dc4befb169be02a0216ba9

@rwldrn

```
Sizes - compared to master @ f717226b3a44f918eec30b2d59ab257270189bc3
    240642       (+79)  dist/jquery.js                                         
    127191       (-31)  dist/jquery.min.map                                    
     83965       (-12)  dist/jquery.min.js                                     
     30003       (-14)  dist/jquery.min.js.gz 
```