Cleanup copyValues DAO function

[colemanw]

Overview

Cleans up function variables and removes a transitional parameter.

Before

• The $serializeArrays parameter was transitional, but would trigger a test failure if it was incorrectly omitted. So at this point if all the tests are passing, it can be removed.
• Unnecessary passing $params by reference.

After

• The $serializeArrays parameter is gone as serialization is now always enabled.
• Passing $params by reference was unnecessary and has been removed.
• Renamed a few variables for readability.

[civibot]

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[colemanw]

@eileenmcnaughton as we saw in #16575 the $serializeArrays param will cause noisy test failures if it is set to false when it should be true. But setting it to true doesn't hurt anything, so let's just get rid of it and treat it as always true.

[eileenmcnaughton]

@colemanw sure - it was just caution & the fact we haven't been seeing those notices tells us there aren't other arrays going in