By Shashwat Chauhan Dec 4 (Reuters) - ETF provider ProShares has withdrawn its registration request for some highly leveraged exchange-traded funds after receiving a warning letter from the U.S. securities regulator that ​flagged risk exposures and paused the review of such plans. Such letters were sent on Tuesday to ‌nine ETF providers, including ProShares, Direxion and GraniteShares, asking them to provide more clarity on the risks tied to funds that sought ‌to track up to fives times the performance o ...