dev: Remove using namespace TheISA in uart8250.cc.

Nothing in that file is in the TheISA namespace, so there's no reason
to use using on it.

Change-Id: I279c27af86509f75ac4e340956381041a0dbcdc4
Reviewed-on: https://gem5-review.googlesource.com/c/13537
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/dev/serial/uart8250.cc

@@ -46,7 +46,6 @@
 #include "mem/packet_access.hh"
 
 using namespace std;
-using namespace TheISA;
 
 void
 Uart8250::processIntrEvent(int intrBit)