CI: Remove uniquely zephyr machines

Remove microbit-v1, qemu-cortex-a53, qemu-cortex-m3, and qemu-cortex-r5
from CI (and the tree in general).  These machines are part of the
meta-zephyr CI now and keeping them here is redundant.  However, keeping
zephyr builds for machines that also have TF-M.

Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -126,12 +126,6 @@ juno:
       - TOOLCHAINS: [gcc, clang]
         FIRMWARE: [uboot, edk2]
 
-microbit-v1:
-  extends: .build
-  parallel:
-    matrix:
-      - TESTING: testimage-zephyr
-
 musca-b1:
   extends: .build
 
@@ -144,18 +138,6 @@ n1sdp:
     matrix:
       - TOOLCHAINS: [gcc, armgcc]
 
-qemu-cortex-a53:
-  extends: .build
-
-qemu-cortex-m3:
-  extends: .build
-  parallel:
-    matrix:
-      - TESTING: testimage-zephyr
-
-qemu-cortex-r5:
-  extends: .build
-
 qemu-generic-arm64:
   extends: .build
   parallel: