Remove backward compatibility code for attachments UI in SMF 2.2

[Sesquipedalian]

From the PR notes for #7676:

Because changes to theme templates in a patch release have a high likelihood of causing problems for existing custom themes, I took extra precautions in order to avoid breaking them. In particular, the changes to smf_fileUpload.js and Post.php include steps to ensure that they remain fully backward compatible with theme files from SMF 2.1.3 and earlier. Those extra tests and fallbacks can safely be removed in SMF 2.2.