fix wasi-nn abi definitions (#4307)

sync with a more appropriate version of the definitions.

as we use the "wasi_ephemeral_nn", which is p1-based, it seems
more appropriate to use definitions from witx, not wit.

it's a bit unfortunate p2-based wasi-nn made gratuitous changes
like this from p1.

note: this is an ABI change.

core/iwasm/libraries/wasi-nn/src/wasi_nn_openvino.c

@@ -161,8 +161,6 @@ wasi_nn_tensor_type_to_openvino_element_type(tensor_type wasi_nn_type)
 #if WASM_ENABLE_WASI_EPHEMERAL_NN != 0
         case fp64:
             return F64;
-        case bf16:
-            return BF16;
         case i64:
             return I64;
         case u8: