remove unused ggml_rope_xpos_back

it is better to have only one `ggml_rope_back` function that accepts all rope parameters, so that `ggml_compute_backward` can propagate all parameters without having to switch between different rope_back variants.

ggml.h

@@ -1380,14 +1380,6 @@ extern "C" {
             float                 beta_fast,
             bool                  beta_slow);
 
-    GGML_API struct ggml_tensor * ggml_rope_xpos_back(
-            struct ggml_context * ctx,
-            struct ggml_tensor  * a,
-            struct ggml_tensor  * b,
-            int                   n_dims,
-            float                 base,
-            bool                  down);
-
     // alibi position embedding
     // in-place, returns view(a)
     GGML_API struct ggml_tensor * ggml_alibi(