mac80211: Remove unused 'struct rate_control_ref' variable

Commit 3b17fbf8 introduced sta_get_expected_throughput()
leaving variable 'struct rate_control_ref* ref' set but unused.
Compiling with W=1 gives the following warning, fix it.

net/mac80211/sta_info.c: In function ‘sta_set_sinfo’:
net/mac80211/sta_info.c:2052:27: warning: variable ‘ref’ set but not used [-Wunused-but-set-variable]

Fixes: 3b17fbf8 ("mac80211: mesh: Add support for HW RC implementation")
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Maxim Altshul <maxim.altshul@ti.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>