[web] Remove newThreadPasswordInput from thread-settings-modal

Summary: As part of cleaning up ThreadSettingsModal found that newThreadPasswordInput wasn't being used anywhere... so removed it.

Test Plan: Searched file (and rest of codebase) for newThreadPasswordInput and newThreadPasswordInputRef and found no usages.

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3726