RFR: 8234532: Remove ThreadLocalAllocBuffer::_fast_refill_waste since it is never set

[Albert Mingkun Yang]

It has some cascading effect; two performance variables, `fastWaste` and `maxFastWaste`, can be removed also.

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Commit messages:
 - refill

Changes: https://git.openjdk.java.net/jdk/pull/3756/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3756&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234532
  Stats: 47 lines in 6 files changed: 0 ins; 40 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3756.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3756/head:pull/3756

PR: https://git.openjdk.java.net/jdk/pull/3756