Bug 23431: Don't nullify DOB if it's hidden by sysprefs

Bug 23431: Don't nullify DOB if it's hidden by sysprefs

To test:

Turn on  OPACPatronDetails
put dateofbirth in PatronSelfModificationBorrowerUnwantedField
Have a patron with a set date of birth
Attempt any modification on that patron through the OPAC
The date of birth will be one of the fields changed for that borrower
when you check it in the staff intranet.

Apply this patch, repeat the above.
Date of birth will not be one of the changed fields.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>