Remove superflous debugging messages from the threading registry.

Remove superflous debugging messages from the threading registry.

These are some random obvious cleanups to the threading dumps, since
it seems I'm not the only one looking at dumps these days.

The "just threaded" debugging message is redundant since there's
already an equivalent "Registering jump thread" message.

The "about to thread" message is actually confusing, because the source
block doesn't match the IL, since the CFG update is mid-flight.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadupdate.c (back_jt_path_registry::adjust_paths_after_duplication):
	Remove superflous debugging message.
	(back_jt_path_registry::duplicate_thread_path): Same.