I have not found exact date of removal from arms of all gliders in the Soviet Army.
Last serial glider of the Soviet Army - Yak-14.
http://www.airwar.ru/enc/glider/yak14.html
In the description it is told that:
"Very soon all of them have replaced on An-12, Mi-4 etc."
But exact date it is not written. I think that indeed, all gliders have removed from arms prior to the beginning of 1960.