qt: Remove left-over mentions of showErrorDialog
* lang/qt/src/qgpgmedecryptjob.cpp, lang/qt/src/qgpgmeimportfromkeyserverjob.cpp, lang/qt/src/qgpgmeimportjob.cpp: Remove obsolete comment. * lang/qt/src/qgpgmekeylistjob.cpp, lang/qt/src/qgpgmelistallkeysjob.cpp: Remove long obsolete, #if 0'ed out definitions of showErrorDialog.
Informing the users about errors is the responsibility of the users of
the QGpgME Job API since a long time.