Bluetooth: TBS: Make GTBS mandatory

Any places that checked for GTBS is now always enabled
as GTBS is mandatory to support, and the Kconfig option
was removed.

Also removed a duplicate Kconfig option for TBS,
BT_TBS_SERVICE_COUNT, as it was unused.

The other Kconfig option BT_TBS_BEARER_COUNT is used
instead exclusively to set it if needed.

Since GTBS can now exist alone, it must also
support having its own calls and other values.
This means that the GTBS and TBS instances now share
the same underlying struct.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>