LONDON (Reuters) – Canadian singer Justin Bieber has canceled one of two concerts scheduled for this week in Portugal, the venue in Lisbon said on its website on Monday. It was not immediately clear whether the cancellation was linked to his collapse on-stage in London last week which forced the teen sensation to take a 20-minute break to receive oxygen and then to stay overnight in hospital. "Due to unforeseen circumstances, Justin Bieber was forced to cancel the second performance in Portugal, March 12," a statement said on the website of the Pavilhao Atlantico. …