[native] remove async in saveENSUserAvatar callback

Summary: In D7547 @ashoat gave some feedback about removing the async in removeUserAvatar and removeThreadAvatar. We should do the same for saveENSUserAvatar

Test Plan: flow and everything still works

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek, atul, ashoat

Differential Revision: https://phab.comm.dev/D7555