Ignore the "what" value of the QGpgME::Job::progress signal

This value contains mostly useless information, e.g. something like "-&12"
or "stdin" or other strings that are not suitable for displaying to users.
The value was anyway already ignored by all final consumers of the signal:
DecryptVerifyFilesDialog::progress, NewResultPage::Private::progress,
and ResultPage::Private::progress, all didn't use the value.

• GnuPG-bug-id: T5478