bluetooth: audio: shell: eliminate `ctx_shell` usage

This change aims to eliminate the dependency on `ctx_shell` in
the Bluetooth `audio/shell/*`, making the code more maintainable.
Replaced `shell_*` functions that depended on `ctx_shell` with
the appropriate `bt_shell_*` functions.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>