nfsd: remove fault injection code

It was an interesting idea but nobody seems to be using it, it's buggy
at this point, and nfs4state.c is already complicated enough without it.
The new nfsd/clients/ code provides some of the same functionality, and
could probably do more if desired.

This feature has been deprecated since 9d60d931 ("Deprecate nfsd
fault injection").

Signed-off-by: J. Bruce Fields <bfields@redhat.com>