Fixing Hlt1DownstreamGammaToEE

More details in the presentation at WP3 of July 15 https://indico.cern.ch/event/1567411/#11-hlt1-conversion-lines

We were relying on the new BW division to remove the prescale and tune the pT cut of this line, but it's unsure if it's going to happen (see !2066)

Here we propose to simply remove the prescale and put a tighter pT cut that should roughly keep the rate as it was.

Detailed checks: https://codimd.web.cern.ch/FQjiGl2vQhyatXvz3XeQGQ