sunrpc: remove __rcu annotation from struct gss_cl_ctx->gc_gss_ctx

Commit 5b22216e (nfs: __rcu annotations) added a __rcu annotation to
the gc_gss_ctx field. I see no rationale for adding that though, as that
field does not seem to be managed via RCU at all.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>