lib/test_rhashtable: Remove set but unused variable 'insert_retries'

Variable 'insert_retries' is not effectively used in the function, so
delete it.

lib/test_rhashtable.c:437:18: warning: variable 'insert_retries' set but not used.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3242


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>