usb: musb: davinci: remove redundant code

The session is cleared in the core whenever musb_platform_disable() is
called, so clearing it in the glue driver *_musb_disable() is redundant.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>