Remove obsolete comment from gdbthread.h

A comment in gdbthread.h refers to a global that no longer exists.

Approved-By: Pedro Alves <pedro@palves.net>

gdb/gdbthread.h

diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index 7135515..d294be6 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -764,7 +764,7 @@ extern int thread_count (process_stratum_target *proc_target);
 /* Return true if we have any thread in any inferior.  */
 extern bool any_thread_p ();
 
-/* Switch context to thread THR.  Also sets the STOP_PC global.  */
+/* Switch context to thread THR.  */
 extern void switch_to_thread (struct thread_info *thr);
 
 /* Switch context to no thread selected.  */