When I try to delete extension it shows me this error. "It is still associated with SC ExtMech Activation record/s ["3"]" So I try to go to SC ExtMech Activation record and remove it from there but it shows me it has depended records so can't delete it. Can I delete those referenced records? SC ExtMech CustomSkin Custom Record Field & SC ExtMech Extensions Active Custom Record Field. If I delete there, then production site will not be affected?

Yes this is normal if in doubt if you have sandbox do it there first but I've done it and everything has been fine