Now that boost has been updated > 1.60 (see #22320), we can simplify how we get
wallet path relative to wallet_dir by using:
boost::filesystem::lexically_relative, removing a TODO.
Test coverage comes from test/functional/wallet_multiwallet.py
I first tried this in #20265 which was my first attempted PR, and funny enough exactly 1 year later I'm opening this one to hopefully finally close this.