[Koha] Use and possible removal of Keyword to MARC mapping config

[Owen Leonard]

> What I've done so far builds on the idea that we could get rid of
> "Keyword to MARC mapping" (fieldmapping table) and rely solely on the
> framework and stored fields like is done for all the other fields

A quick test of the bug worked very well. I think this could be a
great improvement.

For what it's worth, this is what our Keyword to MARC mapping looks
like (MARC21):

subtitle 245 b
subtitle 245 n
subtitle 245 p
subtitle 245 h

  -- Owen

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Athens County Public Libraries
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