CLEANUP: applet: remove unused thread_mask

CLEANUP: applet: remove unused thread_mask

Since 1.9 with commit 673867c35 ("MAJOR: applets: Use tasks, instead
of rolling our own scheduler.") the thread_mask field of the appctx
became unused, but the code hadn't been cleaned for this. The appctx
has its own task and the task's thread_mask is the one to be displayed.

It's worth noting that all calls to appctx_new() pass tid_bit as the
thread_mask. This makes sense, and it could be convenient to decide
that this becomes the norm and to simplify the API.