ramips: switch rt3883 subtarget to kernel 5.4

The target has testing support for kernel 5.4 for quite a while,
compiles fine for all devices, and has been run-tested on Asus
RT-N56U successfully.

Let's set it to kernel 5.4 by default to increase the audience
before an 20.xx stable branch.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Eneas U de Queiroz <cotequeiroz@gmail.com> [Asus RT-N56U]

target/linux/ramips/rt3883/target.mk

@@ -9,8 +9,6 @@ CPU_TYPE:=74kc
 
 DEFAULT_PACKAGES += kmod-rt2800-pci kmod-rt2800-soc wpad-mini swconfig
 
-KERNEL_PATCHVER:=4.14
-
 define Target/Description
 	Build firmware images for Ralink RT3662/RT3883 based boards.
 endef