Remove audio device toggling when ending CallKit calls

[bobiechen-twilio]

Description

The audio-device toggling in the provider:performEndCallAction: delegate method would cause the audio engine to print out error messages like these:

provider:didDeactivateAudioSession:
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: 561017449 (enable 3, outf< 1 ch,  16000 Hz, Int16> inf< 1 ch,  16000 Hz, Int16>)
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: 561017449 (enable 3, outf< 1 ch,  16000 Hz, Int16> inf< 1 ch,  16000 Hz, Int16>)
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: 561017449 (enable 3, outf< 1 ch,  16000 Hz, Int16> inf< 1 ch,  16000 Hz, Int16>)
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: 561017449 (enable 3, outf< 1 ch,  16000 Hz, Int16> inf< 1 ch,  16000 Hz, Int16>)
[aurioc] AURemoteIO.cpp:1086:Initialize: failed: 561017449 (enable 3, outf< 1 ch,  16000 Hz, Int16> inf< 1 ch,  16000 Hz, Int16>)

The audio device only needs to be enabled in the provider:didActivateAudioSession: method. Removing the toggling here fixes the error.

Steps to reproduce

• Ongoing (incoming/outgoing) VoIP call using TwilioVoice
• Make a PSTN incoming call to interrupt the existing call
• Tap "End & Accept" in the system call UI

[paynerc]

Was this change also made to the shared Quickstart repo? Or does this need to be merged here and then in the shared repo?