Remove an extraneous direct use of Time::ParseDate in Articles

When datetime searching was introduced in RTFM 2.0, it could not rely on
RT's core RT::Date field to parse arbitrary times, and took the long way
around, calling Time::ParseDate itself.  This is unnecessary; switch to
the standard ->Set(Format => 'unknown', Value => ...) phrasing.

Fixes I#24498.
Remove an extraneous "use Time::ParseDate"

This came in during 7d367769, which moved it from within the
HTML::Mason::Commands package to the RT::Interface::Web package.  This
had previously made "parsedate" available to Mason elements, but as of
7d367769 it no longer was -- nor is it used in RT::Interface::Web
directly.  Remove it.
Merge branch '4.2/remove-extra-parsedate' into 4.2-trunk