[REF] dev/core#2790 towards pdf task trait

[REF] dev/core#2790 towards pdf task trait

Deprecate CRM_Activity_Form_Task_PDFLetterCommon

This gets us to having 2 classes rather than 3 that manage the activityPDF task
functionality.

CRM_Activity_Form_Task_PDFLetterCommon doesn't really add anything from a structure POV
but it does make it more confusing. There are also functions on the parent
that are only used by this class - which makes switching to a trait
harder. This untangles that part.

Note that once we have the trait (& some more token cleanup done) we will be well placed
to re-share some of these functions again