[OMEMO] Add the feature to delete the OMEMO node

[Neustradamus]

Dear @psi-im team, @psi-plus team, @Ri0n, @Vitozz, @tehnick,

Can you add the feature to delete the OMEMO node?

It is linked to feature: enable/disable OMEMO by XMPP account because after it, the XMPP account must not have OMEMO plugin enabled (no creation of a new device id) but if there are other XMPP accounts in the client, it must not changed for others.

The goal is to remove OMEMO node (all devices...).

<iq id='XXXXXX' from='[email protected]' to='[email protected]' type='get'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<items node='eu.siacs.conversations.axolotl.devicelist'/>
</pubsub>
</iq>

<iq type="result" from="[email protected]" to="[email protected]/client" id="XXXXXX">
<pubsub xmlns="http://jabber.org/protocol/pubsub">
<items node="eu.siacs.conversations.axolotl.devicelist">
<item id="current">
<list xmlns="eu.siacs.conversations.axolotl">
<device id="XXXXXXXXX"/>
<device id="XXXXXXXXXX"/>
</list>
</item>
</items>
</pubsub>
</iq>

cc: @stigger.