[landing] Remove isDev gate for "Launch app" icon in landing Header

Summary: We were gating the icon behind an isDev flag until https://web.comm.app was ready to go, and now it is.

Test Plan: Clicked the button and it took me to https://web.comm.app

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3656