It is possible to remove the built-in apps although they are very small (~1MB) and they are useful. Anyway...
The first time you need to run web2py with all its files. After that you can remove any installed application you want including admin, examples and welcome.
Nothing breaks except that you can no longer visit pages that you do not have. You only get an internal error if you try to do that.
welcome.tar is the scaffold application. That's one file you should not remove.
Notice that if you remove admin you can no longer login as administrator and the database administration (appadmin) will be disabled for all you apps, unless you adit the appadmin.py files and change the authentication mechanism.