Restoring a wallet backup from another chain should obviously result
in a dedicated error message (we have "Wallet files should not be
reused across chains. Restart bitcoind with -walletcrosschain to
override." for that). Unfortunately this is currently not the case
for legacy wallet restores, as in the course of cleaning up the
newly created wallet directory a `filesystem_error` exception is
thrown due to the directory not being empty; the wallet database did
indeed load successfully (otherwise we wouldn't know that the chain doesn't
match) and hence BDB-related files and directories are created in the wallet
directory.

For bitcoind, this leads to a very confusing error message:
```
$ ./src/bitcoin-cli restorewallet test123 ~/.bitcoin/regtest/wallets/regtest_wallet/wallet.dat
error code: -1
error message: filesystem error: in remove: Directory not empty ["/home/thestack/.bitcoin/wallets/test123"]
```

Even worse, the GUI crashes in such a scenario:
```
libc++abi: terminating with uncaught exception of type std::__1::__fs::filesystem::filesystem_error: filesystem error: in remove: Directory not empty ["/home/thestack/.bitcoin/wallets/foobar"]
Abort trap (core dumped)
```

Fix this by simply deleting the whole folder via `fs::remove_all`.