do not configure platform-specific image-tags

Containerd (the container-runtime used to run containers for
concourse-pipelines) is well capable of choosing correct
platform-specific image from "image-index"-manifests (as is the case for
"cc-job-image"). Hence, remove unnecessary logic from pipeline-template,
and thus remove hard-coding assumption about platform-specific
image-tags being set (the latter making migration to
GitHub-Actions-based pipelines slightly easier).

concourse/resources/defaults.mako

@@ -1,5 +1,5 @@
 <%namespace file="/resources/image.mako" import="task_image_resource"/>
-<%def name='task_image_defaults(registry_cfg, platform=None, indent=0)'
+<%def name='task_image_defaults(registry_cfg, indent=0)'
 filter="indent_func(indent),trim">
 <%
 # platform: model.concourse.Platform
@@ -12,9 +12,6 @@ if not (job_image_tag := os.environ.get('CC_JOB_IMAGE_TAG', '')):
   with open(concourse.paths.last_released_tag_file) as f:
     job_image_tag = f.read().strip()
 
-if platform:
-    job_image_tag = f'{job_image_tag}-{platform.normalised_oci_platform_tag_suffix}'
-
 # registry_cfg must be of type ContainerRegistryConfig (cc-utils)
 repository = 'europe-docker.pkg.dev/gardener-project/releases/cicd/job-image'
 registry_cfg = mcr.find_config(image_reference=repository)

concourse/resources/email.mako

@@ -16,7 +16,6 @@ from makoutil import indent_func
 
 concourse_cfg = cfg_set.concourse()
 node_cfg = concourse_cfg.worker_node_cfg
-platform = node_cfg.default_platform
 %>
 - task: '${job_step.name}.failed'
   config:
@@ -28,7 +27,7 @@ platform = node_cfg.default_platform
 % for key, value in env_vars.items():
       ${key}: ${value}
 % endfor
-    ${task_image_defaults(cfg_set.container_registry(), platform=platform, indent=4)}
+    ${task_image_defaults(cfg_set.container_registry(), indent=4)}
     run:
       path: /usr/bin/env
       args:

concourse/templates/default.mako

@@ -376,7 +376,6 @@ else:
 % else:
     ${task_image_defaults(
       registry_cfg=default_container_registry,
-      platform=job_step.platform,
       indent=4,
     )}
 % endif