Newly generated binstubs will check for the string
`This file was generated by Bundler` inside `bin/bundle`, so we'd have
to update that anyway.

Also, there is a non-zero chance the updated `bundle` binstub resolves
some of the setup-specific issues we've seen.

It's… safe to assume that nobody is using this, as this has been broken
for quite some time.

… and drop the single_process_mode. See the included Changelog entry
for full details on what this change means.