gpu-compute,misc: Remove unused private variable

Clang 9 fails to compile GCN3 due to the unused private variable,
`_nxtFreeIdx`, in `src/gpu-compute/dyn_pool_manager.hh`. This variable
has therefore been removed.

Change-Id: I33f2e9634bbf8d5cea7a42ae2ac9f3ea8298d406
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40397
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/gpu-compute/dyn_pool_manager.hh

@@ -46,7 +46,7 @@ class DynPoolManager : public PoolManager
 {
   public:
     DynPoolManager(const PoolManagerParams &p)
-        : PoolManager(p), _regionSize(0), _nxtFreeIdx(0)
+        : PoolManager(p), _regionSize(0)
     {
         _totRegSpaceAvailable = p.pool_size;
     }
@@ -63,8 +63,6 @@ class DynPoolManager : public PoolManager
     // actual size of a region (normalized to the minimum size that can
     // be reserved)
     uint32_t _regionSize;
-    // next index to allocate a region
-    int _nxtFreeIdx;
     // total registers available - across chunks
     uint32_t _totRegSpaceAvailable;
     // regIndex and freeSpace record