This is as simple as it is written. Just uninstall CarbonHeaders :)

The error:

--->  Fetching distfiles for CarbonHeaders
--->  Verifying checksums for CarbonHeaders
--->  Extracting CarbonHeaders
--->  Configuring CarbonHeaders
Error: CarbonHeaders is an obsolete port.  Please uninstall it.
Error: org.macports.configure for port CarbonHeaders returned: obsolete port
Warning: targets not executed for CarbonHeaders: org.macports.install org.macports.configure org.macports.build org.macports.destroot
Please see the log file for port CarbonHeaders for details:
    /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_ports_devel_CarbonHeaders/CarbonHeaders/main.log
Error: Unable to upgrade port: 1
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets

The uninstallation:

florians-MacBook-Pro:~ root# port -v uninstall CarbonHeaders
--->  Deactivating CarbonHeaders @18.1_2
--->  Cleaning CarbonHeaders
--->  Removing work directory for CarbonHeaders
--->  Uninstalling CarbonHeaders @18.1_2
--->  Cleaning CarbonHeaders
--->  Removing work directory for CarbonHeaders

Now, if you run again the update command, you should not get into this error.