misc: Remove lingering uses of TheISA::.

Change-Id: Ie55e0d79867fbc8f75a993fb456a58c84de5def4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62196
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/dev/riscv/plic_device.cc

@@ -40,8 +40,6 @@
 namespace gem5
 {
 
-using namespace RiscvISA;
-
 PlicIntDevice::PlicIntDevice(const Params &params) :
     BasicPioDevice(params, params.pio_size),
     system(params.system),

src/dev/riscv/plic_device.hh

@@ -46,8 +46,6 @@
 namespace gem5
 {
 
-using namespace RiscvISA;
-
 class PlicIntDevice : public BasicPioDevice
 {
   protected: