iwlwifi: mvm: rs: Remove unused 'mcs' variable

Commit 5fc0f76c introduced Rx stats from debugfs, the function
iwl_mvm_reset_frame_stats from that commit defines and sets mcs but
does not use it. Compiling iwlwifi with W=1 gives this warning -

iwlwifi/mvm/rs.c: In function ‘iwl_mvm_update_frame_stats’:
iwlwifi/mvm/rs.c:3074:14: warning: variable ‘mcs’ set but not used [-Wunused-but-set-variable]

Fixes: 5fc0f76c

 (iwlwifi: mvm: add Rx frames statistics via debugfs)
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Cc: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>