Delete these several lines (#9432)
Delete these several lines (#9432)

The `_register_rules` method on `Scheduler` was creating a Python set and adding a key associated with each rule to that set, but then never using it for anything. This commit removes that code.
benjyw/pants,tdyas/pants,tdyas/pants,benjyw/pants,pantsbuild/pants,tdyas/pants,benjyw/pants,benjyw/pants,benjyw/pants,tdyas/pants,tdyas/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,pantsbuild/pants,pantsbuild/pants,tdyas/pants,tdyas/pants,pantsbuild/pants,tdyas/pants,tdyas/pants,jsirois/pants,benjyw/pants,jsirois/pants