usb: f_serial: Remove usage of CONFIG_MODEM_SUPPORT

CONFIG_MODEM_SUPPORT enables interrupt endpoint functionality and
required different set of descriptors with USB Modem interface.
USB Modem interface is required by default anyway, hence remove
usage of CONFIG_MODEM_SUPPORT and enables same functionality by
default.

Change-Id: Ib4b909971201baac1378ce323c23c29bf598688a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>