De-register 5.1.1 packages from Qt Creator during uninstallation

The toolchains aren't removed, since they are shared between different
Qt packages. Ideally they should be registered/unregistered directly
in future MinGW packages.

Qt Creator 2.8.0 or newer is required in for the scripts that check
the return value of the sdktool.

Task-number: QTBUG-32248
Change-Id: Ic98550f65ec60a6117c3e478998d6eb1b1207251
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>