Revert "Add restriction to prevent remove all the lines in a personal expense sheet"

This reverts commit 290c7dfb5b.

This is not needed as there's already an annotation @NotEmpty in
expenseSheetLines. So it's mandatory for all the ExpenseSheet having at least
one ExpenseSheetLine.

FEA: ItEr76S28UserDashboard

libreplan-business/src/main/java/org/libreplan/business/expensesheet/entities/ExpenseSheet.java

@@ -232,13 +232,4 @@ public class ExpenseSheet extends IntegrationEntity implements IHumanIdentifiabl
         return !personal;
     }
 
-    @AssertTrue(message = "a personal expenses must have at least one line")
-    public boolean checkConstraintPersonalExpenseMustHaveAtLeastOneLine() {
-        if (!personal) {
-            return true;
-        }
-
-        return !expenseSheetLines.isEmpty();
-    }
-
 }