soc: qcom: rq_stats: clean up the rq_stats

Remove unncessary function and data structure for rq_stats
since no driver relies on this information. Keep the defer_timer
kobj since irq balancer still needs this.

Change-Id: I20516fd7ed8496426ef75246c7b77c1937c696dc
Signed-off-by: Runmin Wang <runminw@codeaurora.org>