Print pages: don't show custom fields user has no access to

Prior to this, if a custom field's 'read' access level were higher than
the user's access, the print pages would display the field's label (but
not the data). This behavior is not consistent with how this situation
is handled in the View Issue page, where the field is not shown at all.

For consistency, we now skip the of the custom field users have no read
access to in the print pages, so that nothing is displayed.

Fixes #15528