Metal backend: clean up supported_fallback_kernels (#15355)

Removes `aoti_torch_mps_addmm_out` from the `supported_fallback_kernels`
until an implementation is added.

Author: manuelcandales

backends/apple/metal/metal_backend.py

@@ -29,7 +29,6 @@
 
 # exist fallback operators in et namespace;
 supported_fallback_kernels: Dict[str, Any] = {
-    "aoti_torch_mps_addmm_out": None,
     "aoti_torch_mps_convolution": None,
     "aoti_torch_mps_mm_out": None,
     "at::_ops::_scaled_dot_product_attention_math_for_mps::call": None,