target: armv7a: drop command 'cache_config l2x'

target: armv7a: drop command 'cache_config l2x'

The command was already tagged as deprecated in 2015 with commit
0df557728216 ("armv7a: remove l1 flush all data handler") but has
never been removed.
An equivalent command 'cache l2x conf' was introduced at the same
time in commit cd440bd32a12 ("add armv7a_cache handlers").

Drop it and deprecate it.
Replace the old command in the Tcl script.

Change-Id: Ie24eccc99a78786903704d10ee1d9f6c924529b5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8857
Tested-by: jenkins