power: Remove unused variable

Unused after commit:
"power: Ignore interaction boost sequence in a short period"
(I977e844935539a4598a749c77f4c0934ca93a2b7)

Change-Id: I270048ac550c89d292a4dcd470c600c1e8344952
diff --git a/power-common.c b/power-common.c
index 16bf084..deb5e2c 100644
--- a/power-common.c
+++ b/power-common.c
@@ -66,7 +66,6 @@
 
 void process_interaction_hint(void* data) {
     static struct timespec s_previous_boost_timespec;
-    static int s_previous_duration = 0;
     static int prev_interaction_handle = -1;
 
     struct timespec cur_boost_timespec;
@@ -91,7 +90,6 @@
         return;
     }
     s_previous_boost_timespec = cur_boost_timespec;
-    s_previous_duration = duration;
 
     int interaction_handle =
             perf_hint_enable_with_type(VENDOR_HINT_SCROLL_BOOST, duration, SCROLL_VERTICAL);