Revert "CI: use resource groups for qemu build test jobs (MR 6132)"

This reverts commit 0eaba406fe.
Resource groups dramatically reduced CI throughput for these jobs,
blocking pipelines for a really really long time.

.gitlab-ci.yml

@@ -163,19 +163,16 @@ build-aarch64:
 
 build-armv7:
   extends: .build
-  resource_group: qemu
   script:
     - .ci/build-armv7.sh
 
 build-armhf:
   extends: .build
-  resource_group: qemu
   script:
     - .ci/build-armhf.sh
 
 build-riscv64:
   extends: .build
-  resource_group: qemu
   script:
     - .ci/build-riscv64.sh