gdb/remote: remove an out of date comment

A comment above an `if` check was accidentally left in place after
this commit:

  commit ddb3f3d8
  Date:   Tue Mar 19 12:34:34 2024 +0100

      Add "error_message+" feature to qSupported

The comment relates to how 'E.msg' style remote replies are not
supported by every packet, but after the above commit they are
supported in all cases (that call packet_check_result), and the
comment should have been removed.