[OpenCDMi] Remove all unnecessary member variables to save some memory

[VeithMetro]

@HaseenaSainul discovered that the VARIABLE_IS_NOT_USED leads to the build error with buildroot:

FrameworkRPC.cpp:383:69: error: ‘unused’ attribute ignored [-Werror=attributes]
  383 |                     VARIABLE_IS_NOT_USED CDMi::IMediaKeySessionExt* _mediaKeysExt;
      |                                                                     ^~~~~~~~~~~~~
FrameworkRPC.cpp:384:51: error: ‘unused’ attribute ignored [-Werror=attributes]
  384 |                     VARIABLE_IS_NOT_USED uint8_t* _sessionKey;
      |                                                   ^~~~~~~~~~~
FrameworkRPC.cpp:385:51: error: ‘unused’ attribute ignored [-Werror=attributes]
  385 |                     VARIABLE_IS_NOT_USED uint32_t _sessionKeyLength;
      |                                                   ^~~~~~~~~~~~~~~~~
FrameworkRPC.cpp:756:55: error: ‘unused’ attribute ignored [-Werror=attributes]
  756 |                 VARIABLE_IS_NOT_USED mutable uint32_t _refCount;

So let's just removed the unused member variables and save some memory

[HaseenaSainul]

LGTM