Remove asserts from not being able to connect to DBus interface

This causes full crashes if it's unable to connect to a certain dbus interface, which is problematic for porting efforts using Plasma Mobile where ModemManager may not be stable. Since its use is ubiquitous in the shell and settings apps, ModemManager instability can cause the lockscreen, shell or apps to crash.

I do not think it is necessary to have the asserts, as the failed DBus calls get logged already.