strparser: remove redundant variable 'rd_desc'

Variable 'rd_desc' is being assigned but never used,
so can be removed.

fix this clang warning:
net/strparser/strparser.c:411:20: warning: variable ‘rd_desc’ set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>