Pelias forms and the dreaded 'sources' parameter -- part 2

[fpurcell]

So the 'sources' parameter is again being sent down to Pelias -- see #86 and #62 for the last time we got rid of that param.

The Pelias API is pretty particular about sources...and unfortunately one cannot send down a blank value for sources (e.g., if you supply sources, you have to be exact and explicit). For the TriMet MOD / Pelias system, where we have more transit related sources inside of Pelias than we care to list (both here and in our sources config), we don't want the sources param to accompany our geocoder requests. So please either brute-force get rid of this, or provide a config that will prevent the sources param from being part of any calls to Pelias.

FWIW, the api to https://github.com/conveyal/isomorphic-mapzen-search#api around the sources parameter is a bit messed up, since it seemingly doesn't allow one to unset the sources param (just set a value). Pelias works fine (afaik) w/out a sources param. And in fact, I suspect that the default 'sources' are configured on the server by most Pelias users. It's only when one wants to limit the sources (like say for stops or intersections or OA data) that one would actually supply a sources parameter.