Bug 1856611 - Don't force the download of the wpt manifest. r=ahal,jgraham

Downloading the web platform tests manifest can be an expensive operation,
and the current code forces the download regardless of whether it's needed
or not.

Thankfully, the code for deciding if we need to (re-)download the manifest
is already in place, so removing the boolean is all that is needed.

The strategy to re-download is:
  - if any manifest file is missing
  - or if it's too old (given by the rebuild_time variable)
  - or if it's empty

Differential Revision: https://phabricator.services.mozilla.com/D208918

testing/mozbase/moztest/moztest/resolve.py

@@ -886,7 +886,6 @@ class TestResolver(MozbuildObject):
             self.topsrcdir,
             self.topobjdir,
             rebuild=False,
-            download=True,
             config_path=None,
             rewrite_config=True,
             update=True,