DS-2104 Only clear metadata for qualifiers actually present

This is a bug fix for an issue where metadata values could disappear during
submission or in the workflow when a qualdrop input item is placed on a
different page from another input item for the same schema+element. The
previous version of this code processed a page by removing all metadata values
from the item for fields listed on that page. For qualdrop inputs, metadata
values for all qualifiers (including no qualifier) were removed rather than
just those qualifiers actually listed in the qualdrop. This bugfix changes the
code to use the qualifiers from the value pairs of the qualdrop.
Merge pull request #607 from aschweer/DS-2104-disappearing-metadata

DS-2104 Only clear metadata for qualifiers actually present