It didn't do anything, since the dev_compact_gossip_store command was
removed. When we make it do something, it crashes since old_len is 0:
```
gossipd: gossip_store_compact: bad version
gossipd: FATAL SIGNAL 6 (version v25.12rc3-1-g9e6c715-modded)
...
gossipd: backtrace: ./stdlib/abort.c:79 (__GI_abort) 0x7119bd8288fe
gossipd: backtrace: ./assert/assert.c:96 (__assert_fail_base) 0x7119bd82881a
gossipd: backtrace: ./assert/assert.c:105 (__assert_fail) 0x7119bd83b516
gossipd: backtrace: gossipd/gossip_store.c:52 (append_msg) 0x56294de240eb
gossipd: backtrace: gossipd/gossip_store.c:358 (gossip_store_compact) 0x56294
gossipd: backtrace: gossipd/gossip_store.c:395 (gossip_store_new) 0x56294de24
gossipd: backtrace: gossipd/gossmap_manage.c:455 (setup_gossmap) 0x56294de255
gossipd: backtrace: gossipd/gossmap_manage.c:488 (gossmap_manage_new) 0x56294
gossipd: backtrace: gossipd/gossipd.c:400 (gossip_init) 0x56294de22de9
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>