sunrpc: Remove unused function ip_map_lookup

Fix the following clang warnings:

net/sunrpc/svcauth_unix.c:306:30: warning: unused function
'ip_map_lookup' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>