Revert "CI: skip armhf, x86 build on stable branches (MR 2024)"

This does not work as expected, CI doesn't run for x86 and armhf on
master too. An attempt to fix this was made in MR 2030, but it didn't
work as expected and it's not worth looking into this further right now.

This reverts commit d695455f08.

.gitlab-ci.yml

@@ -151,8 +151,6 @@ build-x86_64:
   extends: .build
 
 build-x86:
-  only:
-    - $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
   extends: .build
 
 build-aarch64:
@@ -162,6 +160,4 @@ build-armv7:
   extends: .build
 
 build-armhf:
-  only:
-    - $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
   extends: .build