A senior Hong Kong doctor arrested for allegedly taking cannabis on duty at a public hospital has been released unconditionally after a second urine test revealed no trace of the drug.Dr Yan Yuk-yu, 48, an associate consultant of the accident and emergency department at Tuen Mun Hospital, was arrested at his home in Tai Wai on March 8 after a urine sample tested by the hospital showed he might have taken the substance.But a detailed test by Princess Margaret Hospital on a urine sample taken…