RFR: 8281567: Remove @throws IOException from X509CRLImpl::getExtension docs

[John Jiang]

In class sun.security.x509.X509CRLImpl, method getExtension(ObjectIdentifier) doesn't declare that IOException would be thrown, so the @throws IOException doc should be removed.

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Commit messages:
 - 8281567: Remove @throws IOException from X509CRLImpl::getExtension docs

Changes: https://git.openjdk.java.net/jdk/pull/7419/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7419&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281567
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7419.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7419/head:pull/7419

PR: https://git.openjdk.java.net/jdk/pull/7419