fab: Don't try to delete nonexistent build log

/tmp/docker-pkg-build.log does not appear to be created by docker-pkg
anymore, so don't try to delete it at the end of the operation.

Before:
$ ./fab deploy_devimages
...
== Step 2: publishing ==
Successfully published image docker-registry.discovery.wmnet/dev/fundraising-bullseye-php74-apache2:1.0.2
== Build done! ==
cat: /tmp/docker-pkg-build.log: No such file or directory
and non-zero exit status

After:
$ ./fab deploy_devimages
...
== Step 2: publishing ==
Successfully published image docker-registry.discovery.wmnet/dev/fundraising-bullseye-php74-apache2:1.0.2
== Build done! ==
and zero exit status