[native] Remove unnecessary invariant in ImagePasteModal:sendImage

Summary: I can't see any reason we have an invariant at the end of the callback here. Looks like this was introduced in one of my first diffs (https://phabricator.ashoat.com/D639) and I probably didn't really understand what was happening with flow/invariants

Test Plan: Flow

Reviewers: palys-swm, def-au1t, benschac, varun, ashoat

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3423