RFR: Remove legacy section on mercurial

[Jesper Wilhelmsson]

Mercurial is no longer used in the OpenJDK project.

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Commit messages:
 - Remove legacy section on mercurial

Changes: https://git.openjdk.org/guide/pull/127/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=127&range=00
  Stats: 314 lines in 3 files changed: 15 ins; 296 del; 3 mod
  Patch: https://git.openjdk.org/guide/pull/127.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/127/head:pull/127

PR: https://git.openjdk.org/guide/pull/127