Remove Selfrando from our build system

We don't plan to move forward with Selfrando deployment as it is not much more work for a browser attacker to bypass it, it's not available on all platforms, and it has some issues (like delayed load in e10s mode, see: #26579 (moved)). Additionally, it's work to fix build breakage (in elfutils) to make what we build with compatible with newer GCCs. And I expect another round of Firefox compilation issues when switching to ESR 68.

All in all I think the gains for our alphas are not worth the effort.