Remove testing ONNX from CI (bytecodealliance#10411)

This is blocking CI currently as a downloaded artifact is currently
offline. This can be re-enabled once that's fixed.

Author: alexcrichton

.github/workflows/main.yml

@@ -808,7 +808,7 @@ jobs:
   test_wasi_nn:
     strategy:
       matrix:
-        feature: ["openvino", "onnx"]
+        feature: ["openvino"]
         os: ["ubuntu-latest", "windows-latest"]
         include:
           - os: windows-latest