Disable KActivities integration on Wayland

Once again KActivities causes a delay in the startup of KWin. This is
not the first time it happens, that has been a problem since the start
of the Wayland project.

In the past we added a compile time switch due to that and added a
runtime switch once it stoped freezing (20a9a2a2).
And now it broke again, so let's disable again.

KActivities doesn't make sense on Wayland anyway as there is no activity
support for Wayland windows. Once KActivities is able to be used without
freezing KWin on startup and being able to ensure that it won't break
again we can reenable. But I expect guarantees about that. This must be
ensured by proper autotest support to verify that in future usage of
KActivities in the display manager doesn't freeze it.

This is maintainer decision, due to that no pre-commit review.

CCMAIL: plasma-devel@kde.org
BUG: 388628