Remove unused GMGen randomname i18n keys

[Vest]

Summary

Delete two i18n keys that became dead weight after #7566:

• in_plugin_randomname_name — "Random Name" / "Generador de Nombres" / etc.
• in_mn_plugin_randomname_name — single-letter mnemonic.

Both were only ever referenced from plugin/doomsdaybook/RandomNamePlugin.java, the GMGen plugin wrapper that advertised the "Random Names" tab name and mnemonic to GMGen's plugin host:

private static final String IN_NAME    = "in_plugin_randomname_name";
private static final String IN_NAME_MN = "in_mn_plugin_randomname_name";
...
LanguageBundle.getString(RandomNamePlugin.IN_NAME);
LanguageBundle.getMnemonic(RandomNamePlugin.IN_NAME_MN);

#7566 (Random-name dialog: rewrite from Swing to JavaFX) deleted that wrapper as its first commit ("Remove dead RandomNamePlugin GMGen wrapper"). The live path is now RandomNameDialog → RandomNamePanelController + RandomNamePanel.fxml in core — no plugin tab label, no plugin-style mnemonic, no replacement key needed.

The _plugin_ namespace was specific to GMGen plugin self-identification; core dialogs don't follow that convention.

Verification

• code/src/test/pcgen/gui2/ScanForUnusedIl8nKeysTest flagged the keys as unused after Random-name dialog: rewrite from Swing to JavaFX #7566 (visible in the cleaned.properties / unused.properties snapshots that drift on the working tree after the rewrite).
• Manual grep across *.java, *.fxml, *.lst, and other *.properties (excluding the bundle files themselves and the test snapshots): no references.
• Removed from the four locale bundles that still carried them — LanguageBundle.properties (en), _es, _fr, _it. _de, _ja, _pt already lacked them.
• Regenerated cleaned.properties via ./gradlew slowtest --tests pcgen.gui2.ScanForUnusedIl8nKeysTest. unused.properties is unchanged because the keys are now simply absent from the source bundle.

Test plan

• slowtest --tests pcgen.gui2.ScanForUnusedIl8nKeysTest passes; snapshots up to date.
• Random-name dialog still functional (manually opened from the Summary tab).
• CI green on master merge.