fix plone.restapi to not pass the metadata_fields into the query

[jensens]

plone.restapi's @search passes the entire request dict to the catalog (handler.py#L102), including control parameters such as metadata_fields (e.g. every request from Volto's ObjectBrowserWidget). These are not catalog indexes.

see also bluedynamics/plone-pgcatalog#179

cc @davisagli