Remove `private` and `public` modiifiers in objective-c code

[maratal]

It's not used anywhere in the codebase except two places for reasons unknown to me.

@implementation ARTPushChannelInternal {
@private
    dispatch_queue_t _queue;
    dispatch_queue_t _userQueue;
@public
    __weak ARTRestInternal *_rest; // weak because rest may own self and always outlives it
    ARTInternalLog *_logger;
    __weak ARTChannel *_channel; // weak because channel owns self
}

and

@implementation ARTRestChannelInternal {
@private
    dispatch_queue_t _userQueue;
    ARTRestPresenceInternal *_presence;
    ARTRestAnnotationsInternal *_annotations;
    ARTPushChannelInternal *_pushChannel;
@public
    NSString *_basePath;
}

┆Issue is synchronized with this Jira Task by Unito