delete test_quant_lowering_custom_backend_pass.py as there are no tests in it (pytorch#2809)

Summary:
Pull Request resolved: pytorch#2809

Deleted some tests in this file a while back because they were just testing the old deprecated quant flow. Now I've realized there are no other tests in this file so just deleting the whole thing.

Reviewed By: mergennachin

Differential Revision: D55437765

fbshipit-source-id: 187cc2c66f36b562e7d233065b0394493afafe51

Author: JacobSzwejbka

exir/tests/TARGETS

@@ -226,25 +226,6 @@ python_unittest(
     ],
 )
 
-python_unittest(
-    name = "quant_lowering_custom_backend_pass",
-    srcs = [
-        "test_quant_lowering_custom_backend_pass.py",
-    ],
-    deps = [
-        "//caffe2:test-lib",  # @manual
-        "//caffe2:torch",
-        "//executorch/exir:lib",
-        "//executorch/exir:pass_base",
-        "//executorch/exir/backend:backend_details",
-        "//executorch/exir/backend:compile_spec_schema",
-        "//executorch/exir/backend:partitioner",
-        "//executorch/exir/backend/canonical_partitioners:canonical_partitioner_lib",
-        "//executorch/exir/dialects:lib",
-        "//executorch/exir/passes:replace_aten_with_edge_pass",
-    ],
-)
-
 python_unittest(
     name = "common",
     srcs = [