Drop outdated QT_USE_FAST_OPERATOR_PLUS

Summary:
QT_USE_FAST_OPERATOR_PLUS has been deprecated in Qt 4.8, in favour of
QT_USE_QSTRINGBUILDER. The latter also covers QByteArray.

Qt5 headers still supports QT_USE_FAST_OPERATOR_PLUS (at least 5.11.1),
as subset of QT_USE_QSTRINGBUILDER as it was introduced in Qt4.8,
but not as documented build flag.

Given QT_USE_QSTRINGBUILDER is set here and thus triggers anything that
QT_USE_FAST_OPERATOR_PLUS would trigger in Qt code, removing it should
clean up the macro from undocumented features and thus reduce confusion.

No non-Qt is known at least in KDE repos which checks this build flag
otherwise, so no regression should be expected here as well.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D13800