Remove bug-avoiding StoreConfig * casts for settings

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80431 has been fixed, and
per the previous commit we now check that is the case at build time.

src/libstore/local-store.hh

@@ -40,12 +40,12 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
 {
     using LocalFSStoreConfig::LocalFSStoreConfig;
 
-    Setting<bool> requireSigs{(StoreConfig*) this,
+    Setting<bool> requireSigs{this,
         settings.requireSigs,
         "require-sigs",
         "Whether store paths copied into this store should have a trusted signature."};
 
-    Setting<bool> readOnly{(StoreConfig*) this,
+    Setting<bool> readOnly{this,
         false,
         "read-only",
         R"(