dmaengine: kill some unused headers

The dmaengine redux left some unneeded headers in
include/linux/dmaengine.h, clean them up.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 1956c8d..96e676e 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -23,9 +23,6 @@
 
 #include <linux/device.h>
 #include <linux/uio.h>
-#include <linux/kref.h>
-#include <linux/completion.h>
-#include <linux/rcupdate.h>
 #include <linux/dma-mapping.h>
 
 /**