Make sure all tracks are selected by dragging down them, then select the region in the time ruler you want to remove and use 'Cut special...', make sure that the 'Delete hole' option is checked.
Note: if you only select the time ruler region without selecting all tracks to be affected, it won't work as you require.
[edit] Thinking about it, you might need a dummy clip in the region you want to remove, I'm not quite sure as I've not used this technique on completely empty space before.