regression: Shortcut resetting not possible

[akks]

While working at #6239 I noticed some bug.

How to reproduce:

• (Clean preferences for sure)
• Change shortcut Shift-B (Distribute nodes) to Ctrl-Shift-B in F12
• Restart JOSM
• Check "Use default" checkbox for Distribute nodes, press OK
• Restart JOSM
• Look at shortcuts - Ctrl-Shift-B persists (preferences.xml too).

I thought it was my bug due to prefJPanel GUI code simplification, but it is reproducible since [4897]
(works for [4896], broken in [4897], [4900] helps to save user shortcuts but not reset it to defaut - not working for all later revisions).

After looking at the code I decided that I can not fix it myself.

[stoecker]

In 4916/josm:

fix #7375 - resetting shortcuts not possible