LeAudio: Remove redundant CSIP dependency

Since the set member availability handler was removed from the
service in favor of handling this event in Settings, there is no
point in having the available members list anymore. It's not used
for anything and can be removed.

Bug: 150670922
Tag: #feature
Test: atest BluetoothInstrumentationTests
Sponsor: jpawlowski@
Change-Id: I636bce70fca62f1c4b6e7f9be5d1857158ff3b6c