Map modding is not coding. It's just altering the level entities. It would more so fall under the list of level editing about coding.
As for what to delete, you will want to remove spawnable NPC's. Some triggers which typically either spawn enemies, start up specific soundsets such as blowing wind or outside noises, or spawn scripts. Waypoints are not needed, and also typically you do not need to delete ref_tags since those are usually used for elevators(in most cases). Do not delete the worldspawn either.
This is in reference to the fact that you probably most likely are looking to map mod single player maps(I'm figuring) or B probably the most used MP maps such as ffa_bespin.
In my opinion map modding is a waste of energy. You would be better off adding a completely new and custom map. Leave default maps the way they are. Come up with new interesting things to use and play instead of just rehashing the same old levels(bespin, deathstar, yavin temple) that are already good and fine the way they are. Floating lifts and fancy glowing boons don't improve or make the levels more interesting. Same with throwing random models all over the map. They just turn the level into a giant eyesore