OpenVPN certificate is downloaded from API_BASE, using api_uri field
from provider.json.

This fixes bug #2780
Removed unused methods and variables.

Variables from LeapSRPSession were there because I used it while testing
srp calculations, comparing that strings with the ones from javascript.

Unused method from ProviderAPI was there because I foresee I'll have to
implement it in the future, but I've removed it since it's already in
the history.

This fixes #2781.
We are not using SD storage anymore.

We were using it to save certificate and provider.json files from chosen
provider, so that exporting them was easily done.

We don't need that files, because if we wanted to export that files we
would be able to recover them easily and updated.

This fixes #2783
Removed some unnecessary libraries.

I've also removed some unnecessary comments from LeapHttpClient.
bitmask json files are downloaded.

There was a problem on the assets file "bitmask.url". It had an error in
the eip-service url.

We should use this file only for main url, and proceed as if it were a
new provider but with a preseeded main url.