CLEANUP: config: remove test for impossible case regarding bind thread mask

CLEANUP: config: remove test for impossible case regarding bind thread mask

During 2.5 development, a fallback was implemented for bind "thread"
directives that would not map to existing threads, with commit e3f4d7496
("MEDIUM: config: resolve relative threads on bind lines to absolute ones").
The approch consisted in remapping the threads to other ones. But now
that relative threads and not absolute threads are stored in this mask,
this case cannot happen anymore, and this confusing hack is not needed
anymore.