arm: Delete debug print in initialization of hardware thread

There seems to have been a debug print left in when the original ARMv8
support was merged in. This printout is performed every time you
initialize a hardware thread, and it prints raw pointers, so it always
causes diffs in the regression. This patch removes the debug print.

src/arch/arm/isa.cc

@@ -144,7 +144,6 @@ ISA::ISA(Params *p)
     pmu->setISA(this);
 
     system = dynamic_cast<ArmSystem *>(p->system);
-    DPRINTFN("ISA system set to: %p %p\n", system, p->system);
 
     // Cache system-level properties
     if (FullSystem && system) {