Resolves #2143.

#2141 migrated per-slice marker-style preferences from Slice<N>_MarkerThin (bool) to Slice<N>_MarkerWidth (int 0/1/3). loadDisplayPrefs() reads the old key only as a fallback when the new key is absent, but never deletes it — so the orphan accumulates in ~/.config/AetherSDR/AetherSDR.settings indefinitely.

Change

After migrating the value into m_markerWidth, remove the old key so subsequent saveDisplayPrefs() calls don't carry the orphan forward:

} else {
    // Migrate from the old MarkerThin bool: True (thin) → 1, False (thick) → 3.
    m_markerWidth = (s.value(keyT, "False").toString() == "True") ? 1 : 3;
    if (s.contains(keyT))
        s.remove(keyT);
}

The remove only fires once per slice (the next load will already see keyW and skip the else branch entirely), so this is a one-time cleanup per settings file.

The next saveDisplayPrefs() call (or the first one after this load) writes the new layout without the orphan, matching the issue's "save once on the next opportunity" approach.

Tiny single-file fix as flagged in the issue.