RFR: 8254178: Remove .hgignore

[Aleksey Shipilev]

After migration to Git, this serves no purpose. The file is not updated, does not seem to be used. We can easily remove
it.

There is also .hgtags, but that might have some value for verification, so let's keep it around until Mercurial repos
are completely dead for everyone. .hgtags removal is tracked as JDK-8254318.

-------------

Commit messages:
 - 8254178: Remove .hgignore
 - Revert "8254178: Remove .hgtags and .hgignore"
 - 8254178: Remove .hgtags and .hgignore

Changes: https://git.openjdk.java.net/jdk/pull/549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254178
  Stats: 18 lines in 1 file changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/549.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/549/head:pull/549

PR: https://git.openjdk.java.net/jdk/pull/549