Disambiguate name of static helper from QtQuick1

Change name of static 'qmlEnableDebuggingHelper' variable to disambiguate
it from QtQuick1 variable with the same name.

Task-number: QTBUG-31064

Change-Id: Ia4b0f18a33a383dd4ade1bdb45506304f9066502
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>
Debugger: Remove handling of QT_DECLARATIVE_DEBUG macro

This was added originally to keep older Qt Creator versions working.
However, any newer versions automatically set QT_QML_DEBUG too, and
checking for both actively prevented selectively enabling QtQuick1 or
QtQuick2 debuggers only.

Change-Id: I4d4a984958ef3430ebe05b267bd87185005bbe51
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>