include/linux/phonedev.h: Remove unused header

The header file include/linux/phonedev.h does not seem to be used
anywhere. It was orphaned by 7326446c "Staging: remove telephony
drivers". Remove it.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>