leaudio: Remove not needed flag in the stream_conf

This structure is valid when configuration pointer is valid.
No need for special flag

Bug: 150670922
Test: atest --host  bluetooth_le_audio_client_test bluetooth_le_audio_test
Tag: #feature
sponsor: jpawlowski@
Change-Id: I207547c67cb2c64c04cde3cee878b83d1bed7196