MINOR: applet: remove the thread mask from appctx_new()

MINOR: applet: remove the thread mask from appctx_new()

appctx_new() is exclusively called with tid_bit and it only uses the
mask to pass it to the accompanying task. There is no point requiring
the caller to know about a mask there, nor is there any point in
creating an applet outside of the context of its own thread anyway.
Let's drop this and pass tid_bit to task_new() directly.