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 a5adf445.

Revert "Fix "Session destroyed while still in pool" assertions"
This reverts commit e48a4dac.

Revert "Enable NetworkManager support conditionally"
This reverts commit ca3a5f33.

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