target/arm920t: remove command 'arm920t cp15i' deprecated in v0.4.0

The command 'arm920t cp15i' was marked as deprecated in release
v0.4.0, almost 11 years ago.
We can reasonably expect that in these years any user of OpenOCD
has already migrated to v0.4.0 or to some newer release, thus has
already updated any local/personal script.
There is no run-time warning about the deprecation, but it is
reported in the help and in the documentation.

Drop the command already deprecated in v0.4.0.

Change-Id: I755c4283e13e125558fcd73b15fe20498eae95ca
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6083


Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>