System: Remove 2nd setting of "Id"

Id was already set to "system".

This changes the Id from the Serial number to "system".

Id now matches the last segment of the URL/@odata.id.
Could not find a hard rule to require the Id to match the last segment
of the URL/@data.id but all Redfish mockups match this way, other bmcweb
resources do as well, and redfishtool is easier to use if this is true.

Tested: Validator passes.

curl -k https://${bmc}/redfish/v1/Systems/system/
...
  "Description": "Computer System",
  "Id": "system",
  "IndicatorLED": "Off",
  "Links": {
    "Chassis": [
      {
        "@odata.id": "/redfish/v1/Chassis/chassis"
      }
    ],

Change-Id: Ib2cc34f033accfe6ed6236d512801674153944fc
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/redfish-core/lib/systems.hpp b/redfish-core/lib/systems.hpp
index 81f3107..0d7e6f4 100644
--- a/redfish-core/lib/systems.hpp
+++ b/redfish-core/lib/systems.hpp
@@ -532,8 +532,7 @@
                                             }
                                         }
                                     }
-                                    aResp->res.jsonValue["Id"] =
-                                        aResp->res.jsonValue["SerialNumber"];
+
                                     // Grab the bios version
                                     fw_util::getActiveFwVersion(
                                         aResp, fw_util::biosPurpose,