dax: delete a stale directory pmem

After commit: 83762cb ("dax: Kill DEV_DAX_PMEM_COMPAT") the pmem/
directory is not needed anymore and Makefile changes were made
accordingly in this commit, but there is a Makefile and pmem.c in pmem/
which are now stale and pmem.c is empty, remove them.

Fixes: 83762cb ("dax: Kill DEV_DAX_PMEM_COMPAT")
Suggested-by: Vegard Nossum <[email protected]>
Signed-off-by: Harshit Mogalapalli <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Reviewed-by: Ira Weiny <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Ira Weiny <[email protected]>

Author: harshimogalapalli