Revert "Add 381 to the buffer (You are now an IRC Operator)"

This reverts commit 11e5f7636de76e809e6dd57f46da75843c61be54.

Having this in the raw buffer multiple times is obviously wrong, because you can
use /oper more than once and would get a new 381 numeric each time. So this
would need to use UpdateRawBuffer() instead of AddRawBuffer() (=replace old
entries with the same text).

However, this is still wrong. If you -o yourself, you no longer have oper
status. ZNC wouldn't notice this and still deliver the 381 numeric to new
clients.

Clients which use raw 381 to detect oper status will have to switch to user mode
+o.

Thanks to Han` for reporting this problem.