RFR: 8276067: ZGC: Remove unused function alloc_high_address_at_most()

[Per Liden]

The function ZMemoryManager::alloc_high_address_at_most() is unused and can be removed.

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Commit messages:
 - 8276067: ZGC: Remove unused function alloc_high_address_at_most()

Changes: https://git.openjdk.java.net/jdk/pull/6146/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6146&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276067
  Stats: 26 lines in 2 files changed: 0 ins; 26 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6146.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6146/head:pull/6146

PR: https://git.openjdk.java.net/jdk/pull/6146