[FIX] mail: prevent traceback on reconnecting to call

Before this commit, when reconnecting to a call that was still holding
a participantCard of an outdated rtcSession a crash would occur as
joining a call removes all your previous rtcSessions while generating
a new one while having a `isCausal` on their participantCards.

This commit removes this `isCausal` as the rtcSession of a
channelMember/participantCard should be able to change over time.

closes odoo/odoo#102498

X-original-commit: c903852de3e6d4b99600797c7b426e2e3dfa4bea
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/models/rtc_session.js

@@ -340,7 +340,6 @@ registerModel({
          */
         callParticipantCards: many('CallParticipantCard', {
             inverse: 'rtcSession',
-            isCausal: true,
         }),
         /**
          * States whether there is currently an error with the audio element.