doc: drop mention of not implemented SPI transport

doc: drop mention of not implemented SPI transport

The commit 93f2afa45f4c ("initial "transport" framework") adds a
dedicated chapter in the documentation about a possible SPI
transport for flashing.
This transport has never been part of OpenOCD and should not be
listed in the documentation.

Drop the chapter.

Change-Id: I9b406754399abda4dc7c2f8cf09dd47730a7e1d9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8670
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins