[NETFILTER]: nf_conntrack: remove ASSERT_{READ,WRITE}_LOCK

Remove the usage of ASSERT_READ_LOCK/ASSERT_WRITE_LOCK in nf_conntrack,
it didn't do anything, it was just an empty define and it uglified the code.

Signed-off-by: Martin Josefsson <gandalf@wlug.westbo.se>
Signed-off-by: Patrick McHardy <kaber@trash.net>