iwlwifi: mvm: remove unused sta_id variable in iwl_mvm_change_queue_owner()

Remove the sta_id variable, to avoid this compile warning when
compiling with W=1:

/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/mvm/sta.c: In function ‘iwl_mvm_change_queue_owner’:
/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/mvm/sta.c:871:5: warning: variable ‘sta_id’ set but not used [-Wunused-but-set-variable]
  s8 sta_id;
     ^

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>