RFR: JDK-8294748: Cleanup unneeded references to hg

[Matthias Baesken]

There are still a number of unneeded references to hg, for example

make/autoconf/spec.gmk.in
 
772 HG:=@HG@
 
make/autoconf/basic_tools.m4
 
345 UTIL_LOOKUP_PROGS(HG, hg)

Those can be cleaned up.

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Commit messages:
 - JDK-8294748

Changes: https://git.openjdk.org/jdk/pull/10554/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294748
  Stats: 17 lines in 7 files changed: 0 ins; 9 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/10554.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10554/head:pull/10554

PR: https://git.openjdk.org/jdk/pull/10554