client: In _rmdir, avoid double lookup for dir ino

With the recent changes in fc80c1dc6ee315ae5e039986602ffadba46cb43b,
we only allow setting the inode once on a MetaRequest.  This triggered
a bug in _rmdir(), where the parent dir inode passed in (from doing a
path_walk() and _lookup()) was being set on the MetaRequest, and then
later the _lookup() for the child inode was taking place,
also setting the inode on the MetaRequest.

For rmdir/unlink, the MDS only uses the filepath field of the request
(not the inode or dentry fields), so we remove the code that sets
those fields in the _rmdir() function.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>