usb: phy: Remove the phy-rcar-gen2-usb driver

The phy-rcar-gen2-usb driver, which supports legacy platform data only,
is no longer used since commit a483dcbf ("ARM: shmobile: lager:
Remove legacy board support").

This driver was superseded by the DT-only phy-rcar-gen2 driver, which
was introduced in commit 1233f59f ("phy: Renesas R-Car Gen2 PHY
driver").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Felipe Balbi <balbi@ti.com>