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: NPedro Jardim <jardim.c.pedro@gmail.com>

arch/arm/mach-imx/imx8m/clock_imx8mq.c

@@ -374,28 +374,6 @@ void init_wdog_clk(void)
 	clock_enable(CCGR_WDOG3, 1);
 }
 
-void init_usb_clk(void)
-{
-	if (!is_usb_boot()) {
-		clock_enable(CCGR_USB_CTRL1, 0);
-		clock_enable(CCGR_USB_CTRL2, 0);
-		clock_enable(CCGR_USB_PHY1, 0);
-		clock_enable(CCGR_USB_PHY2, 0);
-		/* 500MHz */
-		clock_set_target_val(USB_BUS_CLK_ROOT, CLK_ROOT_ON |
-				     CLK_ROOT_SOURCE_SEL(1));
-		/* 100MHz */
-		clock_set_target_val(USB_CORE_REF_CLK_ROOT, CLK_ROOT_ON |
-				     CLK_ROOT_SOURCE_SEL(1));
-		/* 100MHz */
-		clock_set_target_val(USB_PHY_REF_CLK_ROOT, CLK_ROOT_ON |
-				     CLK_ROOT_SOURCE_SEL(1));
-		clock_enable(CCGR_USB_CTRL1, 1);
-		clock_enable(CCGR_USB_CTRL2, 1);
-		clock_enable(CCGR_USB_PHY1, 1);
-		clock_enable(CCGR_USB_PHY2, 1);
-	}
-}
 
 void init_nand_clk(void)
 {