refactor: strip legacy lb-alpine AMI build path post-microvm-merge

ELBv2 LB instances now boot direct via microvm kernel+initramfs (the
lb-alpine-3.21.6 system AMI is no longer used). Remove the build/import
scaffolding:

- delete scripts/images/lb.conf + lb-setup.sh (system-image bake config)
- remove lb-alpine catalog entry from spinifex/utils/images.go
- drop LB_IMG download + BM_LB_ALPINE_IMAGE env from e2e-baremetal.yml

Tracks mulga-siv-81.

.github/workflows/e2e-baremetal.yml

@@ -91,15 +91,6 @@ jobs:
             echo "Ubuntu 26.04 cloud image cached ($(du -h "$UBUNTU_IMG" | cut -f1))"
           fi
 
-          LB_IMG="${BM_ISO_CACHE}/lb-alpine-3.21.6-x86_64.raw"
-          if [ ! -f "$LB_IMG" ]; then
-            echo "Downloading lb-alpine image..."
-            sudo curl -fsSL -o "$LB_IMG" \
-              "https://iso.mulgadc.com/system-ami/lb-alpine-3.21.6-x86_64.raw"
-            echo "  Downloaded $(du -h "$LB_IMG" | cut -f1)"
-          else
-            echo "lb-alpine image cached ($(du -h "$LB_IMG" | cut -f1))"
-          fi
 
       - name: Run Bare-Metal E2E
         env:
@@ -107,7 +98,6 @@ jobs:
           BM_SSH_KEY: ${{ runner.temp }}/bm-ci
           BM_EPDU_ENV: ${{ runner.temp }}/epdu.env
           BM_UBUNTU_IMAGE: ${{ env.BM_ISO_CACHE }}/ubuntu-26.04-cloudimg-amd64.img
-          BM_LB_ALPINE_IMAGE: ${{ env.BM_ISO_CACHE }}/lb-alpine-3.21.6-x86_64.raw
           SPX_EXTERNAL_POOL: ${{ env.SPINIFEX_CI_POOL }}
         run: |
           sudo -E mulga/scripts/baremetal/run-baremetal-e2e.sh \