Delete deprecated QNNPack Backend (#3508)

Summary:

This is unsupported and hopefully unused.

Reviewed By: mcr229, doggeral

Differential Revision: D56942466

backends/backends.bzl

@@ -6,7 +6,6 @@ def get_all_cpu_backend_targets():
     """
     return [
         "//executorch/backends/xnnpack:xnnpack_backend",
-        "//executorch/backends/fb/qnnpack:qnnpack_backend",
     ]
 
 def get_all_cpu_aot_and_backend_targets():
@@ -18,6 +17,4 @@ def get_all_cpu_aot_and_backend_targets():
     return [
         "//executorch/backends/xnnpack:xnnpack_preprocess",
         "//executorch/backends/xnnpack/partition:xnnpack_partitioner",
-        "//executorch/backends/fb/qnnpack:qnnpack_preprocess",
-        "//executorch/backends/fb/qnnpack/partition:qnnpack_partitioner",
     ] + get_all_cpu_backend_targets()

exir/_serialize/TARGETS

@@ -11,7 +11,6 @@ cpp_python_extension(
         "bindings.cpp",
     ],
     visibility = [
-        "//executorch/backends/fb/qnnpack/...",
         "//executorch/backends/vulkan/...",
         "//executorch/backends/xnnpack/...",
         "//executorch/sdk/bundled_program/...",

exir/backend/test/demos/TARGETS

@@ -1,30 +1,5 @@
 load("@fbcode_macros//build_defs:python_unittest.bzl", "python_unittest")
 
-python_unittest(
-    name = "test_xnnpack_qnnpack",
-    srcs = [
-        "test_xnnpack_qnnpack.py",
-    ],
-    preload_deps = [
-        "//executorch/kernels/portable:custom_ops_generated_lib",
-        "//executorch/kernels/quantized:custom_ops_generated_lib",
-    ],
-    deps = [
-        "//caffe2:torch",
-        "//executorch/backends/fb/qnnpack:qnnpack_backend",
-        "//executorch/backends/fb/qnnpack:qnnpack_preprocess",
-        "//executorch/backends/fb/qnnpack/partition:qnnpack_partitioner",
-        "//executorch/backends/xnnpack:xnnpack_backend",
-        "//executorch/backends/xnnpack:xnnpack_preprocess",
-        "//executorch/backends/xnnpack/partition:xnnpack_partitioner",
-        "//executorch/exir:lib",
-        "//executorch/exir/backend:backend_api",
-        "//executorch/exir/passes:spec_prop_pass",
-        "//executorch/extension/pybindings:portable_lib",  # @manual
-        "//executorch/extension/pytree:pylib",
-    ],
-)
-
 python_unittest(
     name = "test_delegate_aten_mode",
     srcs = [