client:  Remove unecessary set_inode() in _rmdir()

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 and being set
on the MetaRequest, and then also setting the dir inode on the MetaRequest.
Removing the set_inode() using the parent dir inode resolves this issue.

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