SUNRPC: Remove broken gss_mech_list_pseudoflavors()

Remove gss_mech_list_pseudoflavors() and its callers. This is part of
an unused API, and could leak an RCU reference if it were ever called.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>