ggml : remove redundant set of contexts used field

[danbev]

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 } }:

             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.