lantiq: drop the FRITZ7360SL led-dsl alias

The power LED can not be used for indicating the xDSL line state and
the in system (boot) state at the same time in LEDE.  As soon as the
xDSL Line goes down, the power LED is switched off.

During xDSL handshake the power led would blink with the same or a
similar pattern as during boot.

Both might confuse users.

Signed-off-by: Mathias Kresin <dev@kresin.me>