various: remove ATOMIC_VAR_INIT

the fallback needed it due to the struct wrapper. but the fallback is
now removed so it's no longer needed.

as for standard atomics, it was never really needed either, was useless
and then made obsolete in C17 and removed in C23.

ref: https://gustedt.wordpress.com/2018/08/06/c17-obsoletes-atomic_var_init/
ref: https://en.cppreference.com/w/c/atomic/ATOMIC_VAR_INIT

common/msg.c

@@ -524,7 +524,7 @@ void mp_msg_init(struct mpv_global *global)
     struct mp_log_root *root = talloc_zero(NULL, struct mp_log_root);
     *root = (struct mp_log_root){
         .global = global,
-        .reload_counter = ATOMIC_VAR_INIT(1),
+        .reload_counter = 1,
     };
 
     pthread_mutex_init(&root->lock, NULL);