UI: Fix double confirmation dialog while discarding source change

QDialog::closeEvent() already calls the reject() function, the override
of the former function was actually allowing a first confirmation dialog
which once discarded gets re-openned by QDialog::closeEvent().

Removing most of the closeEvent() override code is enough to fix the issue
since reject() will be called showing the confirmation dialog.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f