Remove caching of template objects in rt-crontool

rt-crontool has cached lookup of the the name of the template ever since
3.8, when the ability to specify a template by name was introduced.  The
template object is modified in memory when ->Parse is called, however,
updating its ->MIMEObj.  This was not a problem because
RT::Action::SendEmail->Prepare calls ->Parse, which clears out its
->MIMEObj.

Unfortunately, a1a4f73e caused Template objects which already had a
MIMEObj to retain them.  In the context of rt-crontool, this causes the
same content (that formed from the first matching ticket) to be sent out
on all tickets whenever RT::Action::SendEmail is used as the target
action.  RT::Action::RecordCorrespondence is unaffected, however, as it
always calls ->Parse.

Remove the unnecessary cache in rt-conjob; looking up a Template objects
per RT::Ticket result is unlikely to result in performance implications,
as it is a small and well-indexed table, and caching repeatedly proves
to be complicated to implement without unexpected side-effects.

Fixes I#29454.
Merge branch '4.2/cron-template-caching' into 4.2-trunk