configs/rock64: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026769723

It has been broken for at least 2 months:

  https://gitlab.com/buildroot.org/buildroot/-/jobs?name=T%3Drock64&kind=BUILD

The old u-boot-2020.07-rc4 no longer builds.

Unless a fix is provided, this defconfig will be removed.

Fixes:

    https://gitlab.com/buildroot.org/buildroot/-/jobs/11026769723

Cc: Michał Łyszczek <michal.lyszczek@bofc.pl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>
(cherry picked from commit 83af501540)
Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>

DEVELOPERS

@@ -2404,8 +2404,6 @@ F:	package/rcw-smarc-sal28/
 
 N:	Michał Łyszczek <michal.lyszczek@bofc.pl>
 F:	board/altera/socrates_cyclone5/
-F:	board/pine64/rock64
-F:	configs/rock64_defconfig
 F:	configs/socrates_cyclone5_defconfig
 F:	package/netifrc/
 F:	package/openrc/