LeAudio: Remove redundant context type

This member variable was set by the state machine
but read only in the unit tests which still work without
it. Therefore it is redundant.

Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test bluetooth_test_broadcaster --no-bazel-mode
Change-Id: Ibed8531d2692bb5eb834177f8b9ceb83aabf0dfa