delete cmd/gokr-packer and cmd/gokr-updater

Users who need to use gokr-packer to migrate an existing setup to the
instance-centric config can use the following command to install the
last version before it was deleted:

go install github.com/gokrazy/tools/cmd/gokr-packer@703a8605028963d13a9d00815ccedfae13f0ce6d

fixes gokrazy/gokrazy#301

Author: stapelberg