RFR: 8296161: [aarch64] Remove unused "pcrel" addressing mode tag

[Kim Barrett]

Please review this trivial change to remove the unused Address::pcrel
enumerator from the Address::mode enum.

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Commit messages:
 - remove unused pcrel mode

Changes: https://git.openjdk.org/jdk/pull/10927/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10927&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296161
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10927.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10927/head:pull/10927

PR: https://git.openjdk.org/jdk/pull/10927