ARM: shmobile: defconfig: Drop useless CONFIG_USB_RCAR_PHY=y

The Renesas R-Car (Gen1) USB PHY driver (which is being removed) does
not support DT, and is thus useless in a Renesas ARM multi-platform
kernel.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>