ggml : remove redundant set of contexts used field (ggml/978)

This commit removes the setting of the `used` field of the contexts in
the global state (g_state) in `ggml_init`.

The motivation for this change is that I believe that this additional
initialization might not be required after the changes in Commit
45fc4fed0b9fb5b1af4a8525cbebb95e11208732 ("sync : latest changes from
whisper.cpp"), which changed the initialization of the contexts field
from `{ 0 }` to `{ { 0 } }`:

```console
             g_state = (struct ggml_state) {
-                /*.contexts =*/ { 0 },
+                /*.contexts =*/ { { 0 } },
             };
```
My understanding is that the `{0}` initialization might not have
zero-initialized all the nested fields in every array element because of
compiler differences, and might have been the reason for having the
explicit setting of the `used` fields to false.