sim: Remove check whether the System port is connected.

The port will report an error if something tries to use it and
it's not connected. If it isn't needed, there's no reason to force
users to hook something up to it just to satisfy the check.

Change-Id: I0668b8a86c8cb323aba51670fb7914d35acc5198
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34815
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/sim/system.cc

@@ -272,14 +272,6 @@ System::~System()
         delete workItemStats[j];
 }
 
-void
-System::init()
-{
-    // check that the system port is connected
-    if (!_systemPort.isConnected())
-        panic("System port on %s is not connected.\n", name());
-}
-
 void
 System::startup()
 {

src/sim/system.hh

@@ -221,11 +221,6 @@ class System : public SimObject, public PCEventScope
         const_iterator end() const { return const_iterator(*this, size()); }
     };
 
-    /**
-     * After all objects have been created and all ports are
-     * connected, check that the system port is connected.
-     */
-    void init() override;
     void startup() override;
 
     /**