(1/3) Remove unnecessary Discard/Quit button

Summary:
Part 1 of 3 for T7841: Revamp buttons on the bottom to solve various usability issues

CCBUG: 386163
FIXED-IN: 18.04

Spectacle's main window doesn't need a discard/quit button, because there's already a user-visible way of closing the window: the titlebar's close button. The escape key does it, too. This button duplicates existing functionality and takes up room that could be used for a "Configure..." button, and has led to user confusion in the past as well (see https://bugs.kde.org/show_bug.cgi?id=386163)

Test Plan:
Tested in KDE Neon:

• Can still quit with the Escape key
• Quitting automatically discards the current screenshot if it has not been saved yet
• There's now more room in the button bar so we can add a visible "Configure..." button

Reviewers: Spectacle, alexeymin

Reviewed By: Spectacle, alexeymin

Subscribers: alexeymin

Differential Revision: https://phabricator.kde.org/D10283