aum-rollback-bootcount: drop amd64 from image types list

It is to test a u-boot feature, so only required on
architectures ARM* and not on amd64.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

test-cases/aum-rollback-bootcount.yaml

@@ -2,7 +2,7 @@ metadata:
   name: aum-rollback-bootcount
   format: "Apertis Test Definition 1.0"
   image-types:
-    fixedfunction: [ armhf, arm64, amd64 ]
+    fixedfunction: [ armhf, arm64 ]
   image-deployment:
     - OSTree
   type: functional
@@ -55,4 +55,4 @@ run:
     - "For Phase 4, power cycle the board just after starting the kernel,
        repeat it for three times to increase bootcount beyond the limit"
     - "Check that bootlimit has been exceeded, by looking at logs"
-    - "Check that after booting the original deployment has been used"
\ No newline at end of file
+    - "Check that after booting the original deployment has been used"