gc: call gc_run unconditionally when consumer is advanced

gc_consumer_unregister and gc_consumer_advance don't call gc_run in case
the consumer in question isn't leftmost. This code was written back when
gc_run was kinda heavy and would call engine/wal callbacks even if it
wouldn't really need to. Today gc_run will bail out shortly, without
making any complex computation, let alone invoking garbage collection
callbacks, in case it has nothing to do so those optimizations are
pointless. Let's remove them.