quota: Kill an unused extern entry form quota.h

Kill an unused extern entry from quota.h
which is leftover of below patch.

[f32764bd: quota: Convert quota statistics to generic percpu_counter]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Signed-off-by: Jan Kara <jack@suse.cz>

include/linux/quota.h

@@ -267,7 +267,6 @@ struct dqstats {
 	struct percpu_counter counter[_DQST_DQSTAT_LAST];
 };
 
-extern struct dqstats *dqstats_pcpu;
 extern struct dqstats dqstats;
 
 static inline void dqstats_inc(unsigned int type)