typo: mixpanel token set twice to the same value

[dimitropoulos]

which, in this case, is an empty string both times. See line 45 where it's initially set.

What does this PR do? fix a bug

Are there breaking changes in this PR? no

Any background context you want to provide? no

Is there parity with the server-side/android/iOS integration components (if applicable)? no

Does this require a new integration setting? If so, please explain how the new setting works no

Links to helpful docs and other external resources n/a