NO such verbiage exist.
The only words in the Constitution that could if mangled by congressional legislation to authorize such action would be;
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb ; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
Even if the congress were to pass legislation to permit such extra judicial killing the person being killed would have to be a member of the armed forces or militia. Which to the best of my knowledge non of the citizens executed by Obamas administration have been.
Presently any member of the armed forces still is entitled to judicial process under The Uniform Code of Military Justice. Congress has the power to alter the USMJ to permit summary execution if they want. It has been legal in the past aboard ship at sea.