staging: rtl8192e: Remove dead code: rtl_dm.[ch]

- Remove unused fields in dig_t structures. Some of them were only
  initialized and never accessed.
- Remove unused enums/macros/defines in rtl_dm.h
- Remove duplicated function declarations
- Remove unused dm_change_dynamic_initgain_thresh() function
- Remove unused dm_shadow_init() function

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>