Fix incorrect return value of PhabricatorCustomField::readValueFromObject()

Summary:
PhabricatorCustomField::readValueFromObject() returns $this while none of its seven child class implementations returns any value.
Make the method return nothing to make PHPStan a bit happier.

Closes T15895

Test Plan: Run static code analysis; read/grep the codebase.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15895

Differential Revision: https://we.phorge.it/D25751