osbuild: remove unused ContainersInputs (note the *s*)

This commit removes the unused ContainersInputs and moves the
`isStageInputs()` function to `ContainersInput` (not the missing
*s* at the end).

The type/function looks like an oversight, I did a `git grep` and
`grep -r` accross our repos and AFAICT the type is not used.

pkg/osbuild/containers_input.go

@@ -46,6 +46,4 @@ func NewContainersInputForSources(containers []container.Spec) ContainersInput {
 	}
 }
 
-type ContainersInputs map[string]ContainersInput
-
-func (c ContainersInputs) isStageInputs() {}
+func (c ContainersInput) isStageInputs() {}