Remove Bugsnag and Google Billing

[asakura42]

Sources have unneeded Bugsnag and Google Billing features, atavisms from original app version. I propose to delete these lines (but don't know how myself, or I'd already send PR)

[niks255]

Google billing is only for playstore builds, it is not included in github builds of the app - neither original nor mine.

Removing bugsnag is pointless, since it is an open-source library and reports are turned off by default.

[asakura42]

Thanks for the answer.