configs/socrates_cyclone5: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026772887

It has been broken for at least 2 months:

  https://gitlab.com/buildroot.org/buildroot/-/jobs?name=T%3Dsocrates_cyclone5&kind=BUILD

It uses the outdated and not even LTS 4.16.7 Linux kernel, which no
longer builds with modern toolchains.

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

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026772887

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 908ce5b8bb)
Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>

DEVELOPERS

@@ -2403,8 +2403,6 @@ F:	package/libavl/
 F:	package/rcw-smarc-sal28/
 
 N:	Michał Łyszczek <michal.lyszczek@bofc.pl>
-F:	board/altera/socrates_cyclone5/
-F:	configs/socrates_cyclone5_defconfig
 F:	package/netifrc/
 F:	package/openrc/
 F:	package/skeleton-init-openrc/