ci: Drop KERNEL_NAME

Only Alpine kernels use the vmlinuz-VARIANT naming scheme, postmarketOS
kernel images are always called vmlinuz.

Since we only use our own kernels now, we can completely get rid of the
variable and always look for vmlinuz.

[ci:skip-vercheck]: the checks can't handle bumping pkgver *and* pkgrel
[ci:skip-build]: takes a long time and already built successfully in CI

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7398>

.ci/build-jobs.yaml.j2

@@ -72,7 +72,7 @@ build:
 # boot. This will not work if your device uses boot images.
 .device-boot-flow-separate-artifacts:
   variables:
-    CI_TRON_KERNEL__URL: "glartifact://${CI_TRON__PMB_BUILD_JOB}/${CI_TRON__PMB_EXPORT_PATH}/${KERNEL_NAME}"
+    CI_TRON_KERNEL__URL: "glartifact://${CI_TRON__PMB_BUILD_JOB}/${CI_TRON__PMB_EXPORT_PATH}/vmlinuz"
     CI_TRON_INITRAMFS__INITRAMFS__URL: "glartifact://${CI_TRON__PMB_BUILD_JOB}/${CI_TRON__PMB_EXPORT_PATH}/initramfs"
 
 # Extends from this job if the target device boots using fastboot and boot images

device/.gitlab-ci/packages/single-runner-defined-in-ci-common-qcom-devicepkg-bootimg.yml.j2

@@ -24,4 +24,3 @@ test-{{ device.name }}:
     CI_TRON_FASTBOOT__TAGS_OFFSET: 0x00000100
     CI_TRON_FASTBOOT__PAGESIZE: 4096
     CI_TRON_KERNEL_CMDLINE__CONSOLE: "console=ttyMSM0,115200"
-    KERNEL_NAME: "vmlinuz"

device/.gitlab-ci/packages/single-runner-defined-in-ci-common-with-vmlinuz-kernel-variant.yml.j2

@@ -3,7 +3,6 @@
 # * only requires a single runner
 # * has its runner tags definition found in ci-common's ci-tron/common.yml
 # * uses CI-tron as a bootloader
-# * has kernel variants following the naming scheme `vmlinuz-$KERNEL_VARIANT`
 #
 # If this is not the case, feel free to copy the content of this file into
 # your package's folder
@@ -15,6 +14,3 @@ test-{{ device.name }}:
     # Extended from build-jobs.yaml.j2
     - .device-boot-flow-separate-artifacts
     - .test-{{ device.name }}
-
-  variables:
-    KERNEL_NAME: vmlinuz-$KERNEL_VARIANT