kirkwood: switch to kernel 5.10

This target has testing support for more than half a year now.
Time to switch.

Signed-off-by: Adrian Schmutzler <[email protected]>

target/linux/kirkwood/Makefile

@@ -10,8 +10,7 @@ BOARDNAME:=Marvell Kirkwood
 FEATURES:=usb nand squashfs ramdisk
 CPU_TYPE:=xscale
 
-KERNEL_PATCHVER:=5.4
-KERNEL_TESTING_PATCHVER:=5.10
+KERNEL_PATCHVER:=5.10
 
 include $(INCLUDE_DIR)/target.mk