BuildStep: Some cleanup

Remove unused BuildStep API.
Remove outdated docs.
Cleanup includes.

Don't repeat custom cancel messages from build/deploy steps,
as in case of cancel action the follow-up message:
"Canceled build/deployment." always appears anyway.

Change-Id: I50b31e0cc688ee66d76a3a1dbe58eb72702112ad
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/ios/iosdeploystep.cpp

@@ -148,8 +148,6 @@ void IosDeployStep::updateDisplayNames()
 
 bool IosDeployStep::init()
 {
-    if (!BuildStep::init())
-        return false;
     m_device = DeviceKitAspect::device(kit());
     auto runConfig = qobject_cast<const IosRunConfiguration *>(
         this->target()->activeRunConfiguration());