Revert parts of "[FEATURE] Unsubscribe settings for forms"

This reverts some parts of commit c3891f28b24e2d63376069e2aa9da7809403ac5c
because the unsubscribe settings will be edited in a new set of forms
and not in the existing "form" froms.
[FEATURE] Configurable unsubscribe forms

The unsubscribe forms can now be configured in the same way as subscribe
forms in the administration area.

Multiple forms can be configured with different settings and a
personalized newsletter can be used for informing the user about the
unsubscription.

Since this feature is not very good tested it needs to be explicitly
enabled in the fontend by setting the constant "TM_USE_UNSUBSCRIBE_FORM"
to to TRUE in tm_lib.inc.php.