RFR: 8373833: "error.cert.not.found" and "error.explicit-sign-no-cert" errors duplicate each other

[Alexey Semenyuk]

Remove the "error.explicit-sign-no-cert" error message, which duplicates the "error.cert.not.found" error message.

-------------

Commit messages:
 - 8373833: "error.cert.not.found" and "error.explicit-sign-no-cert" errors duplicate each other

Changes: https://git.openjdk.org/jdk/pull/29025/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29025&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373833
  Stats: 17 lines in 3 files changed: 1 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/29025.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29025/head:pull/29025

PR: https://git.openjdk.org/jdk/pull/29025