power: Remove unused private members to fix compile-time warning

Certain versions of clang complain about unused private members if
they are not used. This changeset removes such members from the
POWER-specific ProcessInfo struct to silence the warning.

src/arch/power/stacktrace.cc

@@ -38,7 +38,6 @@ using namespace std;
 namespace PowerISA {
 
 ProcessInfo::ProcessInfo(ThreadContext *_tc)
-    : tc(_tc)
 {
     panic("ProcessInfo constructor not implemented.\n");
 }

src/arch/power/stacktrace.hh

@@ -47,15 +47,6 @@ namespace PowerISA
 
 class ProcessInfo
 {
-  private:
-    ThreadContext *tc;
-
-    int thread_info_size;
-    int task_struct_size;
-    int task_off;
-    int pid_off;
-    int name_off;
-
   public:
     ProcessInfo(ThreadContext *_tc);