update_manager: Don't set the std_err_callback for async git commands

When no callback is set for stderr, the outputfrom stderr is piped to stdout.  This makes setting the callback redundant.

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

moonraker/components/update_manager.py

@@ -1349,7 +1349,6 @@ class GitRepo:
         git_cmd = f"git -C {self.git_path} {cmd}"
         scmd = self.cmd_helper.build_shell_command(
             git_cmd, callback=self._handle_process_output,
-            std_err_callback=self._handle_process_output,
             env=env)
         while retries:
             self.git_messages.clear()