(maint) stop enforcing the random prioritizer for evaluation in spectests

Before 95ddc530 the integration tests for Type::User purging authorized
SSH keys did not work when using the SequentialPrioritizer that is
actually the default in the transaction runner methods defined in
PuppetSpec::Compiler.

Since the cause has been removed, so should we do with the workaround.
The tests should rely on the defaults and not impose a (random) preference
unnecessarily.