osbuild: remove tar image size

Image size should not be specified for a tar image. When the tar
assembler was being created image size was not passed to it. However,
the TarAssemblerOptions contained a Size field which would default to 0.
This field is removed so that the tar assembler options no longer
specify a size. The assembler tests are also updated to reflect this
change.

internal/osbuild/tar_assembler.go

@@ -6,7 +6,6 @@ package osbuild
 // compression type, and stores the output with the given filename.
 type TarAssemblerOptions struct {
 	Filename    string `json:"filename"`
-	Size        uint64 `json:"size"`
 	Compression string `json:"compression,omitempty"`
 }