RFR: 8254255: Remove obsolete .hgignore file

[Kevin Rushforth]

Simple patch to remove the obsolete `.hgignore` file.

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Commit messages:
 - 8254255: Remove obsolete .hgignore file

Changes: https://git.openjdk.java.net/jfx/pull/316/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=316&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254255
  Stats: 73 lines in 1 file changed: 0 ins; 73 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/316.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/316/head:pull/316

PR: https://git.openjdk.java.net/jfx/pull/316