RFR: 8266499: Delete dead code in aarch64.ad

Tue May 4 15:48:08 UTC 2021

Just dead code, nothing to see here.

I had to change a few `MacroAssembler` to `C2_MacroAssembler` in
ad_encode.m4, which seems not to have been updated when 8241436 was
committed.

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Commit messages:
 - 8266499: AArch64: Delete dead code in aarch64.ad

Changes: https://git.openjdk.java.net/jdk/pull/3860/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3860&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266499
  Stats: 54 lines in 2 files changed: 0 ins; 50 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3860.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3860/head:pull/3860

PR: https://git.openjdk.java.net/jdk/pull/3860