When building container images an extra create_fstab method
exists that created an empty fstab file. The reasons for this
are historical and also related to appx support from obs.
The obs based support for appx containers was based on the
creation of a kiwi docker image that got modified and turned
into an appx container. Now with native appx support by kiwi
this special fstab handling is no longer needed and should
be deleted. This Fixes#1329
