Remove unused codepath for single-file tools. NFC (#1013)

We don't have any tools that are just one file, and indeed we
assume these days that tools install to their own directory.

emsdk.py

@@ -1968,11 +1968,7 @@ class Tool(object):
     elif url.endswith(ARCHIVE_SUFFIXES):
       success = download_and_unzip(url, self.installation_path(), filename_prefix=getattr(self, 'zipfile_prefix', ''))
     else:
-      dst_file = download_file(urljoin(emsdk_packages_url, self.download_url()), self.installation_path())
-      if dst_file:
-        success = True
-      else:
-        success = False
+      assert False
 
     if not success:
       exit_with_error("installation failed!")