Remove arm64 jobs from GitLab CI

The only arm64 runner we have at our disposal is suffering from
intermittent connectivity issues which make it unusable for extended
periods of time.  Remove arm64 jobs from GitLab CI until we manage to
set up an arm64 runner with more reliable connectivity.

.gitlab-ci.yml

@@ -54,11 +54,6 @@ stages:
     - linux
     - amd64
 
-.linux-arm64: &linux_arm64
-  tags:
-    - linux
-    - arm64
-
 .linux-i386: &linux_i386
   tags:
     - linux
@@ -96,10 +91,6 @@ stages:
   image: "$CI_REGISTRY_IMAGE:debian-sid-amd64"
   <<: *linux_amd64
 
-.debian-sid-arm64: &debian_sid_arm64_image
-  image: "$CI_REGISTRY_IMAGE:debian-sid-arm64"
-  <<: *linux_arm64
-
 .debian-sid-i386: &debian_sid_i386_image
   image: "$CI_REGISTRY_IMAGE:debian-sid-i386"
   <<: *linux_i386
@@ -743,30 +734,6 @@ unit:gcc:tarball:
   only:
     - tags
 
-# Jobs for regular GCC builds on Debian "sid" (arm64)
-
-gcc:sid:arm64:
-  variables:
-    CC: gcc
-    CFLAGS: "${CFLAGS_COMMON} -Og"
-    EXTRA_CONFIGURE: "--with-libidn2"
-  <<: *debian_sid_arm64_image
-  <<: *build_job
-
-system:gcc:sid:arm64:
-  <<: *debian_sid_arm64_image
-  <<: *system_test_job
-  needs:
-    - job: gcc:sid:arm64
-      artifacts: true
-
-unit:gcc:sid:arm64:
-  <<: *debian_sid_arm64_image
-  <<: *unit_test_job
-  needs:
-    - job: gcc:sid:arm64
-      artifacts: true
-
 # Jobs for regular GCC builds on Debian "sid" (i386)
 
 gcc:sid:i386: