RFR: 8311489: Remove unused dirent_md files

[Daniel Jeliński]

The methods declared in dirent_md are not used in libjava and not exported. They should be removed.

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Commit messages:
 - Remove dirent_md

Changes: https://git.openjdk.org/jdk/pull/14772/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14772&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311489
  Stats: 194 lines in 3 files changed: 0 ins; 193 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14772.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14772/head:pull/14772

PR: https://git.openjdk.org/jdk/pull/14772