Remove permission checks on the system SecurityManager

Remove SecurityManager#check*(..) calls whose receiver is System.getSecurityManager(), directly or through a local variable that only ever holds its result. Since Java 24 there is no system SecurityManager, so these checks are dead code. Checks on any other SecurityManager, such as a parameter, a field or a custom subclass, are kept: such an instance still enforces its own policy, and removing the check would silently allow every action. Blocks left empty and SecurityManager locals left unused are removed afterwards.