secmem: Remove getting cap_ipc_lock by capabilities support.

* src/secmem.c (lock_pool_pages): Remove escalation of the capability.

With CAP_SETPCAP, it might make sense before Linux 2.6.24 when file
capabilityes were not supported. But not any more.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>