Bluetooth: OTC: Remove old ots internal header file

This commit is the first in a series of commits to make the OTC
(Object Transfer Client) implementation that exists in the audio
folder "official". That is, the client implementation will be updated
and refactored to be aligned with the OTS (Object Transfer Service)
implementation that exists in subsys/bluetooth/services/ots, and moved
to that folder.

Preparations for making OTC "official":
Move remaining content of old ots_internal header fil into the
otc_internal header file, and remove the ots_internal header file.
(Another ots_internal header file already exists in the ots folder.)

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>