Revert "Added JsSIP.Session#isActive() method."
Revert "Added JsSIP.Session#isActive() method."

This reverts commit c88fbe2ba156aa472a8a362a48b49d6e1a0e5a6a.

josephfrazier/SIP.js,AaronZhangL/JsSIP,CodeYellowBV/JsSIP,markandrus/SIP.js,vconsult/JsSIP,dpocock/JsSIP,rusekr/JsSIP,onsip/SIP.js,voxbone/JsSIP,ronlobo/SIP.js,Escaux/SIP.js,joseph-onsip/SIP.js,DrumTechnologiesLtd/JsSIP,Voiceworks/SIP.js,tqcenglish/JsSIP,onsip/SIP.js