[REF] Cleanup CustomField BAO to use its own isSerialized function

[colemanw]

Overview

Yet another refactor in a series that is getting way too long and redundant, because the code that handles custom data is way too long and redundant.

Before

Same problem as everywhere else.

After

Same solution as everywhere else.

[civibot]

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[seamuslee001]

This looks fine to me merging