Remove GetDataDir(bool fNetSpecific = true) function
Summary:
This concludes backport of core#21850 [6/6]
https://github.com/bitcoin/bitcoin/pull/21850/commits/aca0e5dcdb174ef7e88b76910d6fffd633688235
Depends on D11956
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D11957