cache: remove drainManager because it's not used

the cache drainManager is set but never cleared, this is because
the cache itself does not need to be drained and thus never
triggers a signalDrainDone(). because the drainManager variable
is not used properly and does not appear to be necessary it has
been removed with this patch.

src/mem/cache/base.cc

@@ -77,7 +77,6 @@ BaseCache::BaseCache(const Params *p)
       blocked(0),
       noTargetMSHR(NULL),
       missCount(p->max_miss_count),
-      drainManager(NULL),
       addrRanges(p->addr_ranges.begin(), p->addr_ranges.end()),
       system(p->system)
 {
@@ -752,8 +751,6 @@ BaseCache::drain(DrainManager *dm)
 
     // Set status
     if (count != 0) {
-        drainManager = dm;
-
         setDrainState(Drainable::Draining);
         DPRINTF(Drain, "Cache not drained\n");
         return count;

src/mem/cache/base.hh

@@ -287,9 +287,6 @@ class BaseCache : public MemObject
     /** The number of misses to trigger an exit event. */
     Counter missCount;
 
-    /** The drain event. */
-    DrainManager *drainManager;
-
     /**
      * The address range to which the cache responds on the CPU side.
      * Normally this is all possible memory addresses. */