Function dr_unregister_pre_syscall_event 

pub unsafe extern "C" fn dr_unregister_pre_syscall_event(
    func: Option<unsafe extern "C" fn(drcontext: *mut c_void, sysnum: c_int) -> bool_>,
) -> bool_

Unregister a callback function for the pre-syscall event. \return true if unregistration is successful and false if it is not (e.g., \p func was not registered).