Revert "Fix call to isWindowPrivate"

This reverts commit 53f4696d9ea1a67103a25537d80194808bd201dc.

This fix breaks support for Firefox 31 (currently ESR).
Merge pull request #628 from fmarier/fx31fix

Revert "Fix call to isWindowPrivate"
Remove old contributeData pref migration
Remove the data sharing option from the UI
Stop mentioning data contribution in the first run screen
Remove the contributeData pref
Remove upload code