Stop linking MPS into the prebuilt pip wheel

We haven't tested this, and we'd prefer to have both MPS and Core ML
working. Remove it for now, putting macOS and Linux on equal footing.

Author: dbort

build/packaging/env_var_script_m1.sh

@@ -21,8 +21,7 @@ CMAKE_ARGS="${CMAKE_ARGS} -DEXECUTORCH_BUILD_XNNPACK=ON"
 
 # When building for macOS, link additional backends into the pybindings runtime.
 
-# TODO(dbort): Core ML requires features only available in macOS 10.15, but the
+# TODO(dbort): Make these build properly in the CI environment.
 # build machine uses an older version.
 # CMAKE_ARGS="${CMAKE_ARGS} -DEXECUTORCH_BUILD_COREML=ON"
-
-CMAKE_ARGS="${CMAKE_ARGS} -DEXECUTORCH_BUILD_MPS=ON"
+# CMAKE_ARGS="${CMAKE_ARGS} -DEXECUTORCH_BUILD_MPS=ON"