target/riscv/pmp: Remove outdated comment

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 10387eec21d2f17c499a78fdba85280cab4dd27f.1618812899.git.alistair.francis@wdc.com

target/riscv/pmp.c

@@ -19,10 +19,6 @@
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-/*
- * PMP (Physical Memory Protection) is as-of-yet unused and needs testing.
- */
-
 #include "qemu/osdep.h"
 #include "qemu/log.h"
 #include "qapi/error.h"