core: remove generic job completion messages from unit vtables

These units' message format strings are identical to the generic
strings. Since we can always rely on the fallback, these are now
redundant.

src/core/slice.c

@@ -297,7 +297,6 @@ const UnitVTable slice_vtable = {
         .status_message_formats = {
                 .finished_start_job = {
                         [JOB_DONE]       = "Created slice %s.",
-                        [JOB_DEPENDENCY] = "Dependency failed for %s.",
                 },
                 .finished_stop_job = {
                         [JOB_DONE]       = "Removed slice %s.",