target/arm720t: remove command 'arm720t cp15' deprecated in v0.4.0

target/arm720t: remove command 'arm720t cp15' deprecated in v0.4.0

The command 'arm720t cp15' 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: I2b325d0312d96ca5e5f0f1bad13bb162b3b75c52
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6082
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>