doxygen: Remove some stale doxygen references.

doxygen: Remove some stale doxygen references.

When we switched to lopsub, the callback request functions became
unused and were removed. At the same time, all command handlers were
made static, so they are no longer part of the doxygen documentation
and we shouldn't refer to them any more.

Also struct slot_info and the server_uptime variable have been made
private to their respective files audiod and time.c. Tthe latter was
in fact replaced by a timeval struct long ago.

The http and udp receivers don't have an init function any more, and
get_chunk_table_of_row() was removed ten years ago in commit 54a480ae.

Most references of the source code documentation will be annotated
with \ref in a subsequent commit to avoid this kind of documentation
bug in the future.