clock_imx8mq: Delete not used init_usb_clk()

Function init_usb_clk() is not used anywhere, so it can simply be deleted.
This was detected by the following sparse warning:

arch/arm/mach-imx/imx8m/clock_imx8mq.c:377:6: warning: no previous prototype
 for ‘init_usb_clk’ [-Wmissing-prototypes]
 void init_usb_clk(void)
      ^~~~~~~~~~~~

Signed-off-by: Pedro Jardim <jardim.c.pedro@gmail.com>