staging: remove rts5139 driver code

Roger writes:
	Since all patches have been applied and the device is now
	supported by the new driver, would you remove the former staging
	one at drivers/staging/rts5139?

Cc: Roger Tseng <rogerable@realtek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>