librbd/cache/pwl: remove RBD_FEATURE_DIRTY_CACHE check in DiscardRequest

librbd/cache/pwl: remove RBD_FEATURE_DIRTY_CACHE check in DiscardRequest

"m_image_ctx.features &&RBD_FEATURE_DIRTY_CACHE" is obviously wrong
because it would pretty much always be true.  However, even if bitwise
AND was used, this check would still be dead because DiscardRequest is
only invoked if RBD_FEATURE_DIRTY_CACHE is enabled:

  int invalidate_cache(ImageCtx *ictx) {
  {
    ...
    // Delete writeback cache if it is not initialized
    if ((!ictx->exclusive_lock ||
         !ictx->exclusive_lock->is_lock_owner()) &&
	ictx->test_features(RBD_FEATURE_DIRTY_CACHE)) {
      C_SaferCond ctx3;
      ictx->plugin_registry->discard(&ctx3);
      r = ctx3.wait();
    }

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>