CI: remove build at from CI:

AT is a standalone application. It will not keep up with latest IDF. If
IDF changes APIs which AT used, CI will fail. Therefore remove AT job.

.gitlab-ci.yml

@@ -109,15 +109,6 @@ build_ssc:
     - git checkout ${CI_COMMIT_REF_NAME} || echo "Using SSC default branch..."
     - MAKEFLAGS= ./gen_misc_ng.sh
 
-build_at:
-  <<: *build_template
-  script:
-    - git clone $GITLAB_SSH_SERVER/application/esp-at.git
-    - cd esp-at
-    - git checkout ${CI_COMMIT_REF_NAME} || echo "Using esp-at default branch..."
-    - make defconfig
-    - make
-
 build_esp_idf_tests:
   <<: *build_template
   artifacts: