Remove the orphaned vendored SafeVerify project

The comparator migration dropped the scorer Dockerfile stage that built
safe_verify, so apn/lean/safeverify/ is now dead: nothing in the Dockerfile,
apn/, scripts/, or tests/ references it. The disproof certifier
(CertifyDisproof.lean) replicates negateExpr's mkNot∘cleanupAnnotations
independently, so no live code depends on the vendored source. Removing it
per plan §5 (drop the scorer stage and the vendored-safeverify COPY/build).

Author: tadamcz