Hide the TBB logo in Tor Launcher

As explained on https://trac.torproject.org/projects/tor/ticket/14122, the TBB logo in Tor Launcher may be confusing in the context of Tails. As seen on that ticket, an environment variable now allows to hide it. Shall we do that?

Feature Branch: feature/9031-tor-browser-4.5

Related issues

Has duplicate #6894 (closed)
Blocked by #9031 (closed)

Original created by @intrigeri on 8696 (Redmine)

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