Fix lint issues in additional test files

- Remove unused imports in test_silicon_platform_build.py
- Remove unused imports in test_sim_platform.py
- Remove unused imports in test_software_generator.py

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: robtaylor

tests/test_silicon_platform_build.py

@@ -6,9 +6,7 @@
 from unittest import mock
 
 import tomli
-from amaranth import Module, Signal
-
-from chipflow_lib import ChipFlowError
+from amaranth import Module
 
 
 class TestSiliconPlatformBuild(unittest.TestCase):

tests/test_sim_platform.py

@@ -6,10 +6,7 @@
 from unittest import mock
 
 import tomli
-from amaranth import Module, Signal, Cat, ClockDomain
-from amaranth.lib import io
-
-from chipflow_lib import ChipFlowError
+from amaranth import Signal
 
 
 class TestSimPlatform(unittest.TestCase):

tests/test_software_generator.py

@@ -2,8 +2,6 @@
 import os
 import tempfile
 import unittest
-from pathlib import Path
-from unittest import mock
 
 from chipflow_lib.software.soft_gen import SoftwareGenerator