NAIROBI, Kenya — All the gunmen who staged a deadly attack on a luxury hotel and shopping complex in Nairobi were killed, Kenya’s president said Wednesday, declaring an end to the brazen overnight siege that underscored the ability of al-Shabab extremists to strike despite military setbacks. Fourteen “innocent lives” were lost in the attack that began on Tuesday, President Uhuru Kenyatta said in a televised address to the nation. “We will seek out every person that was involved in the funding, planning and execution of this heinous act,” Kenyatta vowed in announcing that the all-night operation by security forces to retake the DusitD2 complex was over. Security footage showed at least four heavily armed men in military-style garb took part in the attack, an assault marked by explosions and heavy gunfire.