targets/local: drop Location field

When support for osbuild result was added into osbuild-composer it was in
a bit hacky way - localtarget's location was reused as a path for the
result. This didn't make much sense because we want to store the result
even when image build has no localtarget.

Several past commits made store less dependant on the localtarget. The
responsibility for "holding the paths" to build artifacts was gradually
switched from the localtarget to the store while still maintaining
backwards compatibility - localtarget.Location still pointed at the
correct location.

This commit finishes the switch: local target now has no Location field.
The store is now fully responsible for managing the artifacts and paths
to them. LocalTarget is now just a simple "switch" - if image build has it,
then worker uploads an image into the store and it's then available for
download using the weldr API.

internal/target/local_target.go

@@ -1,7 +1,6 @@
 package target
 
 type LocalTargetOptions struct {
-	Location string `json:"location"`
 }
 
 func (LocalTargetOptions) isTargetOptions() {}