removed excessive escaping

No need to escape room-jid, while leaving, as Strophe.muc.leave() does escaping on its own

see: https://github.com/strophe/strophejs-plugins/blob/5e65dda2f0b406f91afcd4ed2a9124e9a7ad626e/muc/strophe.muc.js#L143 and https://github.com/strophe/strophejs-plugins/blob/5e65dda2f0b406f91afcd4ed2a9124e9a7ad626e/muc/strophe.muc.js#L580