Move SNMPv2-MIB::system from mib.c into the new application_internal.c.

Move SNMPv2-MIB::system from mib.c into the new application_internal.c.
This was the last consumer of mib.c, so send it to the great bitbucket
in the sky.

This move removes the sysORTable, but it was basically non-functioning
anyway and removal of previous entries from the table didn't caused
any fallout that reached my ears.
I hope to add proper sysORTable/agent-capabilities support, which
is compatible with AgentX's {Add,Remove}AgentCaps, soon.

OK tb@