Remove dead code and "current" field from MI thread output doc

Remove dead code and "current" field from MI thread output doc

The MI documentation says that -thread-info output contains a "current"
field in the current thread tuple, with the value "*".  Current GDB
master does not do this, and I couldn't find any GDB version that did.
I suspect that it was never the case.

The code that would correspond to this in print_thread_info_1 is
essentially dead code.  The calls to uiout->text end up in
mi_out::do_text, which is empty.

This patch removes the documentation bit and the dead code.  This
"current" field is not necessary, since -thread-info outputs a
"current-thread-id" field.

gdb/ChangeLog:

	* thread.c (print_thread_info_1): Remove dead code.

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Thread Commands): Remove "current" field
	from -thread-info output.