setup.py: Remove check for bubblewrap

Bubblewrap is not a hard requirement for BuildStream now that it supports other sandboxing mechanisms, i.e. buildbox-run and remote execution.

Aside from that, this check is already a bit misplaced in setup.py. Bubblewrap is a runtime requirement, so warning about it at build time isn't quite right. It makes sense when BuildStream is being installed from source, but not when it is installed from any binary distribution.

Note that the corresponding sandbox implementation already does this check at runtime.