mw.loader: Remove Deferred overhead from execute() hot code path
This was introduced by
0ac4f99804c for the concept of globally
implied legacy dependencies.
Commit
ba257035b07 then re-purposed this for the site/user module
dependency.
The legacy wait was removed in
0ac6076b4c0, thus leaving only the
site/user code.
This commit now simplifies the code back to how it was before
0ac4f99804c
(but keeping the improved error handling from
ba257035b07).
Change-Id: I2eae3b78fbe4f03a7a098d8a6233bdc2b79171b8