BluetoothMidiService: Remove target version

Platform APIs should not explicitly define the API version.
In Android.bp, platform_apis is set to true so the correct API
version will be chosen automatically.

Bug: 215743611
Change-Id: Ie974cbb41a1c2cb2d59991970d3d632a831c4462
Test: Verified Echo and Identity tests with MIDIMatrix
2 files changed