chore(ci): drop build-shim.yml after dropping build required-check

Branch protection on main no longer requires the `build` context:

    $ gh api -X DELETE .../branches/main/protection/required_status_checks/contexts \
        -f 'contexts[]=build'
    ["OSV-Scanner (SCA)", "Trivy (filesystem + container scan)",
     "Semgrep (SAST)", "Gitleaks (secret scan)",
     "jscpd (duplication < 3% on touched code)",
     "SBOM (SPDX + CycloneDX)"]

The shim existed only to satisfy that legacy required check after the
old ci-java.yml's `build` job got deleted in the Phase 6 cutover. The
real Go gates (go-ci.yml + perf-gate.yml) cover what `build` used to.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: aksOps