Remove mostly-duplicate code for Rules which can never trigger

PreviewScrips includes a mostly-duplicated block of code to preview
emails produced by Rules, the mechanism behind Approvals.  However, this
code is unnecessary because /Ticket/Update.html is not accessible for
tickets of type 'approval' (the only objects which could currently
trigger Rules in a stock RT), because /Tickets/autohandler redirects all
but a small number of files under /Tickets/ to /Approvals/ instead.
/Ticket/Update.html is not in the whitelist, and thus cannot render for
Approvals.

Furthermore, the code checks $rule->{hints}{class} to determine if it
should display the rule; no rule ever has a ->{hints} set on it, and
never has, since the introduction of this code in 6d14f535.  As such,
even if /Ticket/Update.html were accessible, or some local customization
made use of Rules for something other than Approvals, they would still
not appear in PreviewScrips.

Remove the duplicated and unused code.
Merge branch '4.2/no-preview-rules' into 4.2-trunk