staging: delete ti-st from staging

The 2 drivers originally staged, the core ti-st driver
and the btwilink bluetooth driver have now moved to relevant
directories, so deleting the ti-st/ from staging.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>