Remove python script for generating dark mode CSS

Now devtools_paths.js exists, we can use JS and remove the Python
version completely. This unblocks further work to enforce that
`.darkmode.css` files are up to date as part of a PRESUBMIT.

Bug: chromium:1164945
Change-Id: Ie9e8fa1cd5287d8e444085d7f036a4b17c99eda9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2662976
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
diff --git a/scripts/devtools_paths.js b/scripts/devtools_paths.js
index 116b8b1..9c77695 100644
--- a/scripts/devtools_paths.js
+++ b/scripts/devtools_paths.js
@@ -145,10 +145,20 @@
   return path.join(nodeModulesPath(), 'stylelint', 'bin', 'stylelint.js');
 }
 
+function downloadedChromeBinaryPath() {
+  const paths = {
+    'linux': path.join('chrome-linux', 'chrome'),
+    'darwin': path.join('chrome', 'chrome-mac', 'Chromium.app', 'Contents', 'MacOS', 'Chromium'),
+    'win32': path.join('chrome-win', 'chrome.exe'),
+  };
+  return path.join(thirdPartyPath(), 'chrome', paths[os.platform()]);
+}
+
 module.exports = {
   thirdPartyPath,
   nodePath,
   devtoolsRootPath,
   nodeModulesPath,
-  stylelintExecutablePath
+  stylelintExecutablePath,
+  downloadedChromeBinaryPath
 };