Removes one level of unnecessary indirection, which aided in finding one issue in #30866. Simplifies the code one step further than https://github.com/bitcoin/bitcoin/pull/30866/commits/09a1875ad8cddeb17c19af34b8282d37fed0937e belonging to aforementioned PR.
No observed difference when running benchmarks: ExpandDescriptor/WalletIsMineDescriptors/WalletIsMineMigratedDescriptors/WalletLoadingDescriptors.