Remove the legacy wallet and BDB dependency
The final step of #20160.
A bare minimum of legacy wallet code is kept in order to perform wallet migration. Migration of legacy wallets uses the independent BDB parser and a minimal LegacyDataSPKM that allows the legacy data to be loaded so that the migration can be completed.
All tests which tested legacy wallet behavior have been removed. The --descriptors and --legacy-wallet options are removed from the functional tests.
BDB has been removed as a dependency and documentation have been updated to reflect that.
Depends on #26596, https://github.com/bitcoin-core/gui/pull/824, #30265, #30328, and #31250