xwayland: Make launcher not force EGL_PLATFORM

It was added to work around Xwayland crashing at startup due to a mesa
issue, see commit 58fa92e4.

It's been almost 6 years since that change. No other major wayland
compositor forces EGL_PLATFORM, so lets remove the workaround.

The main motivation behind this change is to get rid of code that's not
straightforward and should be unnecessary in general.