_toolchain.cxx: remove.

This is causing issues on Python 3.12 and in any case should be
based on the Python `ziglang` package instead of this cursed
setuptools hack.

tests/test_toolchain_cxx.py

@@ -1,68 +0,0 @@
-import os
-import ctypes
-import tempfile
-import unittest
-
-from amaranth._toolchain.cxx import *
-
-
-class ToolchainCxxTestCase(unittest.TestCase):
-    def setUp(self):
-        self.include_dir = None
-        self.build_dir = None
-
-    def tearDown(self):
-        if self.include_dir:
-            self.include_dir.cleanup()
-        if self.build_dir:
-            self.build_dir.cleanup()
-
-    def test_filename(self):
-        self.build_dir, filename = build_cxx(
-            cxx_sources={"test.cc": ""},
-            output_name="answer",
-            include_dirs=[],
-            macros=[],
-        )
-        self.assertTrue(filename.startswith("answer"))
-
-    def test_simple(self):
-        self.build_dir, filename = build_cxx(
-            cxx_sources={"test.cc": """
-                extern "C" int answer() { return 42; }
-            """},
-            output_name="answer",
-            include_dirs=[],
-            macros=[],
-        )
-        library = ctypes.cdll.LoadLibrary(os.path.join(self.build_dir.name, filename))
-        self.assertEqual(library.answer(), 42)
-
-    def test_macro(self):
-        self.build_dir, filename = build_cxx(
-            cxx_sources={"test.cc": """
-                extern "C" int answer() { return ANSWER; }
-            """},
-            output_name="answer",
-            include_dirs=[],
-            macros=["ANSWER=42"],
-        )
-        library = ctypes.cdll.LoadLibrary(os.path.join(self.build_dir.name, filename))
-        self.assertEqual(library.answer(), 42)
-
-    def test_include(self):
-        self.include_dir = tempfile.TemporaryDirectory(prefix="amaranth_hxx_")
-        with open(os.path.join(self.include_dir.name, "answer.h"), "w") as f:
-            f.write("#define ANSWER 42")
-
-        self.build_dir, filename = build_cxx(
-            cxx_sources={"test.cc": """
-                #include <answer.h>
-                extern "C" int answer() { return ANSWER; }
-            """},
-            output_name="answer",
-            include_dirs=[self.include_dir.name],
-            macros=[],
-        )
-        library = ctypes.cdll.LoadLibrary(os.path.join(self.build_dir.name, filename))
-        self.assertEqual(library.answer(), 42)