[netlib/fix] Remove redundant member from DgramSocket that breaks the class
[netlib/fix] Remove redundant member from DgramSocket that breaks the class

- "_socket_handle" actually lives in the base class DgramSocketBase
- With the redundant definition some functions (in particular open)
  operate on the handle in the base class and others (bind) use the
  one in the derived

tum-ei-rcs/mart-common,tum-ei-rcs/mart-common