arm: socfpga: sr1500: Remove USB_GADGET

"buildman" tool revealed that USB_GADGET was enabled by mistake for this
board in process of moving that option to Kconfig. Remove it to bring
things back to correct state.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Stefan Roese <sr@denx.de>

configs/socfpga_sr1500_defconfig

@@ -22,5 +22,3 @@ CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_SYS_NS16550=y
 CONFIG_CADENCE_QSPI=y
-CONFIG_USB=y
-CONFIG_USB_GADGET=y