target: armv7a: remove unused level_num field from armv7a_cachesize

This was never used and produces warnings on some systems.

Change-Id: I48d2c5b79890bb2d70c5fae95278b8eb62743398
Reported-by: Tommy Murphy <tm1234@users.sf.net>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3891
Tested-by: jenkins