lightningd: don't discard const in get_chainparams().

In general, it is true that accessors should take const and discard it,
but chainparams is *always* const.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/lightningd.c

@@ -217,10 +217,9 @@ static void shutdown_subdaemons(struct lightningd *ld)
 	db_commit_transaction(ld->wallet->db);
 }
 
-struct chainparams *get_chainparams(const struct lightningd *ld)
+const struct chainparams *get_chainparams(const struct lightningd *ld)
 {
-	return cast_const(struct chainparams *,
-			  ld->topology->bitcoind->chainparams);
+	return ld->topology->bitcoind->chainparams;
 }
 
 static void init_txfilter(struct wallet *w, struct txfilter *filter)

lightningd/lightningd.h

@@ -169,7 +169,7 @@ struct lightningd {
 #endif /* DEVELOPER */
 };
 
-struct chainparams *get_chainparams(const struct lightningd *ld);
+const struct chainparams *get_chainparams(const struct lightningd *ld);
 
 /* State for performing backtraces. */
 struct backtrace_state *backtrace_state;