RFR: 8295792: Clean up old async close code

[Daniel Jeliński]

Please review this PR that removes the remains of old fdTable-based socket close synchronization.

Verified that tier1-3 tests continue to pass on Linux, MacOS and Windows. Did not verify AIX.

Also removed one slow test for an issue in fdtable implementation; as far as I could tell, it is no longer relevant.

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Commit messages:
 - Clean up after old async close

Changes: https://git.openjdk.org/jdk/pull/10816/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10816&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295792
  Stats: 1159 lines in 12 files changed: 0 ins; 1155 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10816.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10816/head:pull/10816

PR: https://git.openjdk.org/jdk/pull/10816