Remove our 3 custom icons that were relying on the AppStyle in UE5.1

Switched back to what we had before that;
- Ignored => no icon
- Changed => NotInDepot
- LocallyDeleted => Deleted

We cannot do anything else then fully switching to the FRevisionControlStyleManager standard icons because of how the GetIconName() APi is used by the Editor in SSourceControlSubmitWidget::GenerateWidgetForItemAndColumn()

                SNew(SImage)
                .Image(FRevisionControlStyleManager::Get().GetBrush(Item->GetIconName()))
                .ToolTipText(Item->GetIconTooltip())

Source/PlasticSourceControl/Private/PlasticSourceControlState.cpp

@@ -260,23 +260,20 @@ FSlateIcon FPlasticSourceControlState::GetIcon() const
 	case EWorkspaceState::CheckedOutUnchanged:
 	case EWorkspaceState::Replaced: // Merged (waiting for check-in)
 		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.CheckedOut");
-	case EWorkspaceState::Changed: // Changed but unchecked-out file custom color icon
-		return FSlateIcon(FAppStyle::GetAppStyleSetName(), "Plastic.Changed"); // custom
 	case EWorkspaceState::Added:
 	case EWorkspaceState::Copied:
 		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.OpenForAdd");
 	case EWorkspaceState::Moved:
 		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.Branched");
 	case EWorkspaceState::Deleted:
-		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.MarkedForDelete");
 	case EWorkspaceState::LocallyDeleted:
-		return FSlateIcon(FAppStyle::GetAppStyleSetName(), "Plastic.LocallyDeleted"); // custom
+		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.MarkedForDelete");
 	case EWorkspaceState::Conflicted:
 		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.Conflicted");
 	case EWorkspaceState::Private: // Not controlled
+	case EWorkspaceState::Changed: // Changed but unchecked-out is in a certain way not controlled
 		return FSlateIcon(FRevisionControlStyleManager::GetStyleSetName(), "RevisionControl.NotInDepot");
 	case EWorkspaceState::Ignored:
-		return FSlateIcon(FAppStyle::GetAppStyleSetName(), "Plastic.Ignored"); // custom
 	case EWorkspaceState::Unknown:
 	case EWorkspaceState::Controlled: // Unchanged (not checked out) ie no icon
 	default: