sh: sh7786: Remove unused sh7786_usb_use_exclock()

arch/sh/kernel/cpu/sh4a/setup-sh7786.c:411:13: warning: no previous prototype for 'sh7786_usb_use_exclock' [-Wmissing-prototypes]

Upstream never had a user of sh7786_usb_use_exclock(), remove it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/a87c6c0edcf81937b8eb2c899d286d82d71ad513.1709326528.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>