[native] Remove background from modal items

Summary:
Related issue: https://linear.app/comm/issue/ENG-2437/address-feedback-from-design-team-on-light-mode (see discussion in comments)
The background of items in sidebars modal and subchannels modal was beging set to the same value as the modals background in the dark mode, and a different value in the light mode, which resulted in a very weird effect. By removing the
backgroundColor option I'm setting it to be transparent, so now elements have the same background colour as the modal, which is what we want.

This is what used to happen:

This is what happens now:

Test Plan: Run ios (iPhone 13) and android (Android 12) simulators, checked in dark and light mode that in sidebar and subchannels modals items backgrounds look right.

Reviewers: przemek, tomek, michal

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6062