Fix host-interface.hpp build error

Repo CI gives below error:
```
../host-interface.hpp:53:23: error: private field 'bus' is not used [-Werror,-Wunused-private-field]
   53 |     sdbusplus::bus_t& bus;
```

Fix it by removing the unused field.

Change-Id: Idf2c4c2581e79809397cc7327472061432ad9aff
Signed-off-by: Lei YU <yulei.sh@bytedance.com>
diff --git a/host-interface.hpp b/host-interface.hpp
index 52dbab1..5ab587d 100644
--- a/host-interface.hpp
+++ b/host-interface.hpp
@@ -31,8 +31,7 @@
         sdbusplus::server::object_t<
             sdbusplus::server::xyz::openbmc_project::control::Host,
             sdbusplus::server::xyz::openbmc_project::condition::HostFirmware>(
-            bus, objPath),
-        bus(bus)
+            bus, objPath)
     {
         // Nothing to do
     }
@@ -49,9 +48,6 @@
     FirmwareCondition currentFirmwareCondition() const override;
 
   private:
-    /** @brief sdbusplus DBus bus connection. */
-    sdbusplus::bus_t& bus;
-
     /** @brief  Callback function to be invoked by command manager
      *
      *  @detail Conveys the status of the last Host bound command.