libnvdimm/pmem: Remove unused header

'commit a8b456d0 ("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>