Time Preferences crashes with empty selection

Application "Time" crashes when you push "Remove" button without any
selection on list of network time servers.

Fixes #10716

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Signed-off-by: John Scipione <jscipione@gmail.com>
Time: Style fixes to NetworkTimeView
Time: Fix memory leak.

BListView::RemoveItem() doesn't delete the item, it just removes it
from the list. We have to delete the item to free the memory it
consumes ourselves.