Remove 'Need notif permissions' alert from iOS since apple revievers requested to do so

Summary: This differential stops showing "Need notif permissions" alert on iOS but leaves it for android. This change was requested by apple reviewers.

Test Plan: Build app, disable notifications both at the beggining and later in settings and check that alert does not appear in iOS.

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3563