utils/downloads.py: Remove axel downloader

When downloading the official Chromium tar file, axel is no longer able
to parallelize the connections nor detect the filesize. It is likely
Google added a specific rule for axel since curl and Chromium work
normally. While it is possible to circumvent this rule, Google might
have imposed it to reduce spikes of heavy load on their servers. We
should respect this and just use curl instead, which only uses one
connection and has the ability to resume interrupted downloads.

utils/downloads.py

@@ -220,14 +220,7 @@ def _download_if_needed(file_path, url, show_progress, disable_ssl_verification)
         get_logger().debug('Downloading URL %s ...', url)
 
     # Perform download
-    if shutil.which('axel'):
-        get_logger().debug('Using axel')
-        try:
-            subprocess.run(['axel', '-o', str(tmp_file_path), url], check=True)
-        except subprocess.CalledProcessError as exc:
-            get_logger().error('axel failed. Re-run the download command to resume downloading.')
-            raise exc
-    elif shutil.which('curl'):
+    if shutil.which('curl'):
         get_logger().debug('Using curl')
         try:
             subprocess.run(['curl', '-L', '-o', str(tmp_file_path), '-C', '-', url], check=True)
@@ -395,8 +388,8 @@ def main():
         'retrieve',
         help='Retrieve and check download files',
         description=('Retrieves and checks downloads without unpacking. '
-                     'The downloader will attempt to use CLI commands axel or curl. '
-                     'If they are not present, Python\'s urllib will be used. However, only '
+                     'The downloader will attempt to use CLI command "curl". '
+                     'If it is not present, Python\'s urllib will be used. However, only '
                      'the CLI-based downloaders can be resumed if the download is aborted.'))
     _add_common_args(retrieve_parser)
     retrieve_parser.add_argument(