[models] Remove unnecessary Reply-To array length validation
[models] Remove unnecessary Reply-To array length validation

Summary:
I have quite a few emails in my mailbox that have both multiple Reply-To
addresses. This is perfectly OK by the spec.

Fixes: T7369

Test Plan:
regression test coming - making a list and planning to update all the tests once I've hammered out the current crop of fixes I've identified

I also tested and made sure that N1 does the right thing in this case...
multiple Reply-To addresses are displayed correctly, and when you hit "Reply" a
new draft is started with both in the To: field. Makes sense given this is
something the Python sync engine supported too.

Reviewers: jackie

Reviewed By: jackie

Differential Revision: https://phab.nylas.com/D3558

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