On macOS, there are restrictions for root user to prevent crashing your Mac integrity and stability. Removing Safari is risky and useless, that’s why macOS prevents that. Not to mention you don't really save much disk space doing this.
You need to disable System Integrity Protection (SIP). Do this with
csrutil disable command. You can only use it in macOS recovery mode using Terminal.
Do that by rebooting your Mac with Command+R keys held down. there will be a utilities menu, select Terminal and type that command in. When it is done reboot normally.
Do not forget to re-enable SIP with
csrutil enable again in macOS recovery mode once you have deleted the safari.app.