[gui] stop migrating QSettings from BitcoinABC 0.14.6 or Bitcoin Core

Summary:
This is a partial revert of D513. It is highly unlikely in 2024 that anyone will directly upgrade from v0.14.6 to 0.28.12 without any intermediate version. And I don't see a good reason to use Core's settings for a user who first installs Bitcoin ABC.

The affected settings are mostly unimportant GUI state (window geometry, fonts...), and a few parameters that we probably don't want to copy anyway (data dir location...).

("DisplayBitcoinUnit", "MainWindowGeometry", "PeersTabBanlistHeaderState", "PeersTabPeerHeaderState", "RPCConsoleWindowGeometry", "RPCConsoleWindowPeersTabSplitterSizes", "RecentRequestsViewHeaderState", "SubFeeFromAmount", "TransactionViewHeaderState", "UseEmbeddedMonospacedFont", "enable_psbt_controls", "fCoinControlFeatures", "fFeeSectionMinimized", "fHideTrayIcon", "fMinimizeOnClose", "fMinimizeToTray", "fReset", "fRestartRequired", "mask_values", "nConfTarget", "nFeeRadio", "nSettingsVersion", "nSmartFeeSliderPosition", "nTransactionFee", "strDataDir", "strThirdPartyTxUrls")

See https://github.com/Bitcoin-ABC/bitcoin-abc/issues/547 for more context and additional issues solved by this diff.

Test Plan:
build and test bitcoin-qt locally:

cmake .. -GNinja
ninja
src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15682