Use standard Ok/Cancel buttons for the connection removal dialog

Summary:
BUG: 386360

Change the Close button to a Cancel button in the KCM's connection removal dialog.

Test Plan:
Tested in KDE Neon. The dialog now has Ok/Cancel buttons:

Reviewers: Plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8562