nfs: Remove unused variable from write.c

When CONFIG_NFS=y and CONFIG_NFS_V3_{,V4}=n we get the following warning.

	fs/nfs/write.c: In function ‘nfs_writeback_done’:
	fs/nfs/write.c:1246:21: warning: unused variable ‘server’

 Remove the variable 'server' to fix the above warning.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>