Revert the NetworkManager online detection feature

Turns out the 'connectivity' detection (i.e. whether we are behind a
captive portal) does not always work reliably, making resources to
stay offline sometimes even when full connectivity is available.

This feature never reached any stable release.

Revert "Make NMQT dependency optional"
This reverts commit a5adf445616d3d5384f752a26cb9c46172c5315a.

Revert "Fix "Session destroyed while still in pool" assertions"
This reverts commit e48a4dac8a97d08bfe35e329fa08873ea9e96575.

Revert "Enable NetworkManager support conditionally"
This reverts commit ca3a5f33ff02b594ca9964df44ee5619c8afec89.

Revert "Use NetworkManager to detect online status"
This reverts commit 0158097ae239deab8c12584b8e9d0908bf83ac25.