command: drop Jim Command handler, at last

command: drop Jim Command handler, at last

With all OpenOCD commands converted to COMMAND_HANDLER, we can
drop the management of jim_handler commands.
Drop also from documentation the subsection on Jim Command
Registration.

Change-Id: I4d13abc7e384e64ecb155cb40bbbd52bb79ec672
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8901
Tested-by: jenkins