usb: gadget: f_phonet: remove unused preprocessor conditional

The compatibility layer which the USBF_PHONET_INCLUDED was a part of
is no longer present - the USBF_PHONET_INCLUDED is not #defined by anyone
anymore, so the ifndef is always true. Removing it.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>