libnvdimm/pmem: Remove unused header

'commit a8b456d ("bdi: remove BDI_CAP_SYNCHRONOUS_IO")' forgot
remove the related header file.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20201229002635.42555-1-jianpeng.ma@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/pmem.c

@@ -23,7 +23,6 @@
 #include <linux/uio.h>
 #include <linux/dax.h>
 #include <linux/nd.h>
-#include <linux/backing-dev.h>
 #include <linux/mm.h>
 #include <asm/cacheflush.h>
 #include "pmem.h"