[backport#17447] wallet: Make -walletdir network only

Summary:
3c2c439dcd8797019ac6d6614775d5c20ee41c36 wallet: Make -walletdir network only (João Barbosa)

Pull request description:

With this PR `bitcoind -regtest` doesn't run if bitcoin.conf has
```
walletdir=/mnt/mydisk/wallets
```
But works with
```
[regtest]
walletdir=/mnt/mydisk/wallets
```

Doesn't change mainnet behavior.

Closes #15630.

Backport of Core PR17447

Test Plan:

ninja all check check-functional

perform test in the summary

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7713