Obsolete print_bracket_link_prepared()

This function was used in MantisBT 1.x to display links surrounded by
square brackets for menus. With MantisBT Modern UI, it has been replaced
with CSS styling, to display the links as buttons.

One use case was not properly migrated, when displaying links on View
Issues Page (processing the EVENT_MENU_ISSUE hook).

The problem cannot be fixed properly within MantisBT code, it requires
an update of the Plugin's hook (returning an array of label => link
pairs instead of cooked links).

Consequently, the function has been marked as obsolete so that a warning
is displayed, to inform plugin developers that they should update their
code.

Fixes 0023694