rtos: remove display_str member

rtos: remove display_str member

This patch removes the display_str member in the thread_detail struct.
This member was not being used and provides no additional benefit over
the thread_name_str and extra_info_str members. This change is made in
preparation of support for the qXfer:threads:read packet, which will
modernize how thread information is shared with GDB.

Change-Id: I1f8bc6325e6aa790e02ea6caee9d6f44c5fedf36
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/3558
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>