Refactor CRM_Contact_BAO_Contact::importableFields() to use metadata

[colemanw]

Overview

This squares up the recently added metadata with a hardcoded list in the BAO, removing the list in favor of metadata.

Before

Hardcoded list.

After

Uses metadata to do the same thing.

Notes

I compared the hardcoded array to the metadata line-by-line and the only difference I saw was legal_name which I've added to the metadata.

[civibot]

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[eileenmcnaughton]

Looks OK to me