lightningd: cleanup obsolete plugins->shutdown flag

After leaving the main event loop, the only path to destroy_plugin
goes via shutdown_plugins.

lightningd/plugin.h

@@ -113,9 +113,6 @@ struct plugins {
 	/* Blacklist of plugins from --disable-plugin */
 	const char **blacklist;
 
-	/* Whether we are shutting down, blocks db write's */
-	bool shutdown;
-
 	/* Index to show what order they were added in */
 	u64 plugin_idx;