Remove setting of QT_XCB_NO_EVENT_COMPRESSION env var

Summary:
See https://bugreports.qt.io/browse/QTBUG-44964:
The patch that went into Qt 5.6.1 is:
https://codereview.qt-project.org/#/c/157348/
which simply disables compression.

https://codereview.qt-project.org/#/c/156755/ was abandoned, so this
variable never had any effect.

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Subscribers: rempt

Differential Revision: https://phabricator.kde.org/D3066