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 <vegard.nossum@oracle.com>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20241017101144.1654085-1-harshit.m.mogalapalli@oracle.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

Author: harshimogalapalli