afs: Remove set but not used variable 'ret'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/afs/server.c: In function afs_install_server:
fs/afs/server.c:157:6: warning: variable ret set but not used [-Wunused-but-set-variable]

It is not used since commit d2ddc776 ("afs:
Overhaul volume and server record caching and fileserver rotation")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: David Howells <dhowells@redhat.com>