Local police are not required to enforce federal law, essentially. They can, if they want -- it's the highest law in the land -- but the obligation is on the Federal Gov to enforce their own laws, hence the FBI, DEA, ATF, etc.
"Decriminalizing" just means the local police will not enforce the law as written, and if the state or Feddy'Gov doesn't like it then they can send state troopers or DEA agents to hang out on every block in Denver to make sure the law gets followed and shroomers get nailed. Contrary to popular belief, the FBI and DEA don't have unlimited resources, and have bigger fish to fry... so nothing happens most of the time.
In the event the local Denver cops do nail you with shrooms, instead of enforcing the law they'll nail you with a fine or something. No idea what the CO State Troopers are up to, or if they'll be acting in Denver against the decrim bill.
A city can't delete a federal law. A city can tell its police department(s) to ignore a federal law (in the sense that it's physically and logically possible, not in the sense of it actually being legal; I'm not sure if the federal government has the power to compel local police forces to enforce federal laws).
