Function drmgr_unregister_cls_field 

pub unsafe extern "C" fn drmgr_unregister_cls_field(
    cb_init_func: Option<unsafe extern "C" fn(drcontext: *mut c_void, new_depth: bool_)>,
    cb_exit_func: Option<unsafe extern "C" fn(drcontext: *mut c_void, thread_exit: bool_)>,
    idx: c_int,
) -> bool_

Frees a previously reserved callback-local storage (cls) slot index and unregisters its event callbacks. Returns false if the slot was not actually reserved.