MOBILE-2838 ws: Don't store POST request promises
MOBILE-2838 ws: Don't store POST request promises

Since MOBILE-2900 POST request promises are not reused in CoreWSProvider.

moodlehq/moodlemobile2,moodlehq/moodlemobile2,dpalou/moodlemobile2,moodlehq/moodlemobile2,reskit/moodlemobile,saman222/moodlemobile2,moodlehq/moodlemobile2,reskit/moodlemobile,moodlehq/moodlemobile2,FMCorz/moodlemobile2,FMCorz/moodlemobile2,saman222/moodlemobile2,saman222/moodlemobile2,saman222/moodlemobile2,reskit/moodlemobile,reskit/moodlemobile,dpalou/moodlemobile2,reskit/moodlemobile,dpalou/moodlemobile2,saman222/moodlemobile2,moodlehq/moodlemobile2,reskit/moodlemobile