Revert "Check if output is disabled in handle_output()"

Revert "Check if output is disabled in handle_output()"

This reverts commit e71c304444dd3070877887d2bb5407cd64033946.

It turns out that several users have workflows in which they turn off
their monitors without using e.g. `xrandr --output DP-1 --off`. The
result is that the monitors are disconnected, but not disabled.

With commit e71c304444dd3070877887d2bb5407cd64033946, i3 started to see
these two states as one and the same state, but that causes more harm
than it does good. For example, for some users with only one monitor, i3
would just exit when these users turned off their monitor.

related to #1858, #1839

fixes #1845