LeAudioService: Remove dead code

With following change:

05250672 LeAudio: Add support for mute/unmute

There is no need for special PTS mute handling. It can be easly done by
moving volume slider.

Bug: 273276001
Tag: #feature
Test: atest BluetoothInstrumentationTests
Change-Id: I1b4981db448eeb0b73ff2ea97730b91e84f1d6ce