ci: remove realtek-rtl838x

Target was removed from Gluon, see
https://github.com/freifunk-gluon/gluon/pull/3461/commits/4e847f7eef8ce143afd8ade72b56d9f49013e414

.gitlab-ci.yml

@@ -96,7 +96,6 @@ build-target:
       - TARGET: ramips-mt7620
       - TARGET: ramips-mt7621
       - TARGET: ramips-mt76x8
-      - TARGET: realtek-rtl838x
       - TARGET: rockchip-armv8
       - TARGET: sunxi-cortexa7
       - TARGET: x86-generic