Which district had the most number of first degree murders? Give the district number.
the most number refers to max(count(case_number)); first degree murder refers to secondary_description = 'FIRST DEGREE MURDER'; district number refers to district_no
SELECT T2.district_no FROM IUCR AS T1 INNER JOIN Crime AS T2 ON T1.iucr_no = T2.iucr_no WHERE T1.secondary_description = 'FIRST DEGREE MURDER' GROUP BY T2.district_no ORDER BY COUNT(*) DESC LIMIT 1