Remove gnupg_import_ok_pattern from torbrowser_launcher/common.py (#12)

According to https://github.com/torproject/torbrowser-launcher/pull/716
the definition of `gnupg_import_ok_pattern` in
`torbrowser_launcher/common.py` is causing some warnings.

But it looks like it is not being used since
83fa1d38, so we can remove it.

Thanks to meator for reporting the issue.