RFR: 8279903: Redundant modulo operation in ECDHKeyAgreement

[John Jiang]

In class `sun.security.ec.ECDHKeyAgreement.java`, the last `mod()` in the below line looks redundant,

BigInteger lhs = y.modPow(BigInteger.valueOf(2), p).mod(p);

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Commit messages:
 - 8279903: Redundant modulo operation in ECDHKeyAgreement

Changes: https://git.openjdk.java.net/jdk/pull/7042/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7042&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279903
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7042.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7042/head:pull/7042

PR: https://git.openjdk.java.net/jdk/pull/7042