remove manual limitation of coredump size

fix #598 (closed)

it's already disabled by secmem_proc. Specifically:

• we call harden_process_std_err
• which calls harden_process_other_err
• which calls disable_core_dumps
• which calls set_coredump_rlimit
• which calls set_rlimit
• which finally calls libc::setrlimit

(also remove rlimit from arti-testing is it did not really depends on it)