nfsd: remove unused function

Fix the following clang warning:

fs/nfsd/nfs4state.c:6276:1: warning: unused function 'end_offset'
[-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>