coredump: get rid of a bogus assertion

coredump: get rid of a bogus assertion

The check looks plausible, but when I started checking whether it needs
to be lowered for the recent changes, I realized that it doesn't make
much sense.

context_parse_iovw() is called from a few places, e.g.:
- process_socket(), where the other side controls the contents of the
  message. We already do other checks on the correctness of the message
  and this assert is not needed.
- gather_pid_metadata_from_argv(), which is called after
  inserting MESSAGE_ID= and PRIORITY= into the array, so there is no
  direct relation between _META_ARGV_MAX and the number of args in the
  iovw.
- gather_pid_metadata_from_procfs(), where we insert a bazillion fields,
  but without any relation to _META_ARGV_MAX.

Since we already separately check if the required stuff was set, drop this
misleading check.