Remove some code in TagManager and spec (#3547)

* Do not fall back to StreamEntry if object_type is unavailable in TagManager

Since 6d6a429a, when Status, the only model
with stream_entry, and StreamEntry got its own logic in uri_for and
url_for, the purpose of the fallbacks to activity_type of StreamEntry
became unclear.

This commit removes the fallbacks. When adding another model with
stream_entry in future, consider to update uri_for and url_for.

* Cover TagManager more