Bug 40318: Remove check for DISPLAY env var in start-tor-browser
When using Wayland, the DISPLAY environment variable does not need to be set, so the start-tor-browser script should not fail in that case.
Wayland support is still disabled by default in Firefox, and users who want to enable it will need to set the MOZ_ENABLE_WAYLAND environment variable.
Closes #40318 (closed).
/cc @Cortex65