Take into account the deletion of entries on a combobox in more cases

In the "Host" comboBox on the "New Network Connection" dialog: if e.g. the user
has deleted a host address from the url list, that's taken into account even if
afterwards a connection to a host is not made.

Press Ctrl+N to open the "New Network Connection" dialog. Using the "Host"
comboBox on the "New Network Connection" dialog, several times: add a new host
address (e.g. a non-existent one like "example.com"), press the «Connect»
button. Open the "New Network Connection" dialog again, delete the recently
added host address:
a) using Shift+Del when the popup list is seen;
b) or using the arrow keys and Shift+Del when the popup list is not visible;

Try to delete all the host addresses until the list is empty, press Shift+Del
again. Press the «Cancel» button.

Open the "New Network Connection" dialog again, check that the list is empty.

Start a new connection (e.g. an FTP one), just in case.

Note: Those changes are designed to be tried after applying the ones
described in https://phabricator.kde.org/D29000