CompactApplet: Remove redundant check of attached property

Attached properties are created by QML engine on demand, so the mere
check for `Boolean(fullRepresentation.Layout)` would make one if it
didn't exist yet. Currently, unlike in C++, there's no way in pure QML
to check for existence of an attached property. Such check is redundant
in this case, since the property is likely to be set by an applet
anyway.